I filtered it again and let it dry in the filter. It looks greenish so I think it's fertilizer which has recrystallized.
We have an older home with a low pitched rubber roof. What we called in my day, a modern style. However, whatever is on the roof makes the runoff icky and I can't collect it. That rainsaucer idea was terrific and I think I can adapt that attractively.
I'm going to put some old tights over the end of the downpipe to filter out any particles from the roof. Although the water did look very clean without!
Little black speckles? Maybe, if you have asphalt composition shingles on your house, they are the tiny little pieces of "grit" on those. One problem with getting rain water off of roofs is that any thing on the tile goes into the rainbarrel too. People who actually drink rain water have been known to have "sheds" which they then roof with pvc type roofing that does not shed any particle matter, and keeps the water clean. This water goes into large buried cisterns.
I get shingle "pellets" in my rain buckets/cans. They never cause a problem in the years I've been doing it. I transfer a lot of it into 1 gallon milk jugs and use a strainer to filter out any debris. Mainly for my carnivorous plants but I also use it to water my houseplants which include some orchids. My rainwater TDS is usually around 2ppm.