Black rot is caused by several organisms, fungal and non-fungal (Oomycetes). Not many fungicides kill it. The effective ones are extremely expensive.
I suggest you contact your county agricultural extension service for suggestions.
Did you see this link?
PP260/PP260: Black Rot of Orchids Caused by Phytophthora palmivora and Phytophthora cactorum
From a practical standpoint, you are in the situation of a surgeon in the pre-antibiotic era, or a bacteriologist working with harmful pathogens. Your only hope is to cut off healthy pieces of orchid, without contaminating them, sanitize and and re-establish them.
Each seemingly healthy plant also has to be sanitized. Some of these will be infected, but you won't know it yet. I would start with the seemingly healthy plants. After sanitizing them, I would start on the obviously sick plants.
The water mold has contaminated your entire growing area, so that has to be sanitized, as well. 10% household bleach is the most readily-available and effective agent. It will not be very effective on unpainted wood surfaces, though. If you have unpainted wood benches, consider discarding them. It will not be possible to get rid of black rot organisms lingering in damp wood without a long stay in an autoclave.
I looked through the archives and didn't see any complete descriptions of how to treat plants for water mold without cross-contamination. Here is one method that would work. It is extremely labor-intensive. The dipping in bleach is hard on plants, and the treatment will kill some of them. I have never had water mold, but this is based on how I did things when I worked in a bacteriology lab.
Get a box of 1-sided razor blades and a roll of fresh paper towels or napkins. Get some powdered cinnamon, which is an effective antimicrobial agent. Get some disposable gloves and some alcohol hand sanitizer. You can find the gloves at a restaurant supply house.
You will throw away or sanitize each blade after 1 cut (not 1 plant, 1 cut.) Alternatively, have an alcohol or gas lamp to heat your blade between cuts. You can use a gas burner if you have a gas stove. If you will be heating the blade, have something long to hold it so you don't burn yourself.
Get a bucket large enough to hold any one of the plants, and some household bleach. In the bucket put a 10% solution of bleach, 1 part bleach to 9 parts of clean water. Make up a smaller container of 1:10 bleach solution for cleaning your work space. If you want to try and save some expensive pots, make up another large bucket of bleach solution, for pots only.
Prepare a temporary place to hold the orchids you have treated, large enough for them to not touch each other in any way. Wipe down the surfaces with a paper towel very wet with bleach solution and allow them to air dry.
Have an area where you can perform surgery. This area needs to have surfaces you can sanitize with 10% bleach solution before you start, and between each two plants you treat. Wipe this area down with a paper towel and the bleach solution. Get it quite wet.
Take out some blades, dip them in bleach and set them on the wet surface. Let the surface air dry before you put anything else on the surface.
Put the paper towels and gloves near your work area.
Have a large trash container with a liner next to the work area.
Set a small piece of paper towel on the dry work area, and put some cinnamon on the towel, in which to dip the cut end of the plant.
Sanitize your hands with alcohol and let them air dry, without touching anything else. Put on a new pair of gloves. Sanitize your hands right after taking off a pair of gloves, and right before putting on a new pair.
Pick up one seemingly non-affected plant. Try not to let the plant touch any part of you beyond the gloves. If it touches your sleeves you will need to change clothes later, before picking up the next plant.
Take the plant to your outside trash barrel. Unpot the plant into the trash. Remove all the old potting medium and dead plant parts you can with your hands.
If it's a cheap pot, throw it away. If it's an expensive pot, submerge it completely into a bucket of 10% bleach solution for pots only, and let it sit overnight, before rinsing. With your hands, or disposable tools, remove all the old medium, and dead plant parts, directly into the trash. Throw away the tools into the trash when done, or put them into a bucket of 10% bleach to soak.
Examine the plant you have just unpotted carefully. If the plant has any sign of rot, you must treat it as though it has rot. See below. If you don't see rot, submerge the plant in a bucket of bleach solution. Leave it for 10 minutes. It will probably float up, but this is OK if you get the whole plant wet. Throw away your gloves, sanitize your hands with alcohol and wait out the 10 minutes.
Some people would use a fresh bucket of bleach solution for each plant. I don't think this is necessary so long as you detect a strong bleach smell.
Sanitize your hands again with alcohol, and let them dry. Put on fresh gloves. Take the plant out of the bleach solution. Carry it to the sanitized holding area. Set it down, not touching any other plants. From this point on plants should never touch until, after a few weeks, you think the problem is over.
Throw away your gloves, and sanitize your hands with alcohol. After air-drying, put on another pair of gloves. Go to your plants and select another seemingly-healthy plant to treat. Repeat the process.
Once you are finished with healthy plants, begin on the sick ones.
Sanitize your hands, air-dry and put on new gloves. Pick up a sick plant. Examine the plant carefully. If any plants have rot invading the growth adjacent to the last growth, throw them away. The new growth is almost certainly already infected, even if it shows no signs, and you will not be able to save it. The rot has probably already infested 1-2 inches / 2-5cm of seemingly healthy rhizome that shows no signs yet. If you don't have more than 2" / 5cm of plant rhizome past the bad area, including a growth capable of making another growth, the plant is already fatally infected, and should be thrown away now. Sadly, this will be most of your plants.
If you think the plant is salvageable, unpot it at the trash barrel and clean off all the old medium, and loose dead plant parts. Once you have removed all the old medium and loose dead plant parts, submerge it in the bleach solution. Leave it for 10 minutes. It will probably float up, but this is OK if you get the whole plant wet. Throw away your gloves, sanitize your hands with alcohol and wait out the 10 minutes.
Sanitize your hands again with alcohol, and let them dry. Put on fresh gloves. Take the plant out of the bleach solution. Carry it to the sanitized work area.
Try to work over the trash barrel, so as not to contaminate the work surface unless you must. Pick up a blade. If possible, make just 1 cut at least 2" / 5cm forward from any signs of rot. Don't try to cut any closer to the obvious rot. If there is nowhere to cut less than 2" / 5cm from an area of rot, it is better to throw the plant away now.
Cut the plant over the trash barrel so the dead part goes right into the trash. After just one cut, drop the blade into the trash barrel, a container of bleach if you want to re-use it, or onto a clean paper towel if you are going to heat treat it.
If you have to make more than one cut, use a fresh blade for each cut.
Take the good piece of plant. Dip just the cut you made in the cinnamon. Now set the piece in the holding area. Be sure plants don't touch each other here.
Throw away the paper towel with cinnamon. Throw away your gloves. Sanitize your hands with alcohol and let them air dry. Put on fresh gloves. Wipe down the surgery area with a fresh bleach-dipped paper towel, and let it air dry. Heat treat the blade, if desired. Take off your gloves and wash your hands with rubbing alcohol again. Put down a fresh paper towel and cinnamon, and blades, if needed. Put on new gloves. Go pick up another sick plant, and do it again.
When all the plants have been treated, sanitize your work area. Your growing area should be empty of plants. Wash it down with a 10% bleach solution. Cover all surfaces. Let them air dry.
Let the treated plants air dry for a day. If you have plants that can't go a day without watering, pick them up with new gloved hands and dip them in a container of water. Set them back in the holding area without dripping on other plants. Dispose of the water. Dispose of the container, or sanitize it with 10% bleach.
Then pot them up, one by one, into sanitized pots and new medium. Sanitize your hands with alcohol between each plant, and wear new gloves for each plant. Set the plants in your sanitized growing area, but be sure they don't touch. To water, take each plant to the sink. Water with boiled water, or water from new or sanitized containers. Let the plant drain and leaves dry. Then take it back to the growing area without dripping any water on any other plant. Sanitize the sink with 10% bleach solution between plants, and let it air dry.
If the black rot does not come back after a week of this, the problem is over for now. You can water the plants in place, but be careful not to get any water from one plant onto another plant. Be careful there is no standing water anywhere.
Good luck. Unless I had irreplaceable plants, I would throw away all the infected ones and start over, rather than doing this.
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