Spotty new leaf & black coloration at base of mini Phalaenopsis

[Dimples]

I agree, it needs a much larger pot. Get all the roots into the new pot. New bark shouldn’t hold enough water for the aerial roots to have issues with the transfer. Given good light and the heating mat, it should start growing new roots soon.

[Dalachin]

If your home is cool and dry, you can also try adding a liner pot. Repot in a larger pot that will hold all the roots, then drop that into a slightly larger container without holes and put it on a heating mat. That will create a microclimate around the roots that will encourage growth.

I often use plastic containers from my recycling as liner pots, and you can also get decorative ceramic ones.

[Mountaineer370]

I'm in agreement that that plant needs a larger pot and you should try to get as many of those aerial roots as possible into the pot. As others have mentioned, by using large bark, they should be able to adapt.

I do want to add one thing. Many of my Phals have abundant aerial roots that are higher on the plant, between leaves or otherwise growing in such a manner that there's no pushing them into the media when I repot. I live in Michigan, so we have the furnace on in the wintertime, and my indoor humidity has been in the teens. So each and every time I water, I spray the exposed roots thoroughly, aerial roots and those close enough to the top that I can see them through the media. If I didn't do that, all of the exposed roots on my plants would have withered up by now.

One more thing, and maybe someone has mentioned this already. It sounds like the plant has been in sphagnum moss for a long time, and you have now repotted it into chunky bark. Be prepared to water a lot more that previously! Because of the extremely low wintertime humidity here, I am watering about every other day. Once in a while I can go every three days, but if I do that, I have to mist the aerial roots sooner than that. I'm not sure where you live, so you may not be dealing with the same kind of wintertime weather, but with time and experience, and by reading up on the Phal thread linked earlier, you will learn how to gauge when the plant needs watering in its new media and other ways to keep your plant happy in your particular conditions.

[Arizona Jeanie]

One other suggestion:
I would trim off the very bottom of the stem, below where the roots are emerging. This will make it a lot easier to get those roots into the pot.
When making the change from moss to new bark, water-water-water. Even daily. The plant will adapt, but the new bark doesn't hold much water, so it can be flooded every day or two without risking root rot.