Have you reciently added fish or plants? Sometimes I know that algae spores or fungi can come on plastic or live plants. Since the algae eater is completely ignoring the plants, it might be something other than algea afecting your tank. Algae usually isnt black like that either. I agree, go get the water tested, that will tell you the PH, amonia level etc. However, that wont tell you what the black stuff is. Perhaps you should scrape some off or take a column or plant to a fish store (a pet shop that only deals with fish, chain pet stores generally don't have fish "experts" like the exclusive stores do). Id have the water testing done at a fish place too. Hopefully you have a fish shop in your area. Later, I'll ask my dad *Mr fish breeder expert* and see what he thinks.

Good luck!!