Ok, how old are they, and can you run a fresh set tomorrow? If your FC and Cya never deviated from this...and from a time the pool was completely Algae free, you won't have black Algae. But we know that isn't the case, especially if that is some type of Algae. When you say the Chlorine is good, and post old results we don't have anything to work with.
Is it anywhere else in the pool? Somewhere you can touch it, and pinch some of maybe, or smudge it? If so, tell us what happens. What happens when you brush it with a normal brush? Black Algae grows when there is a lack of ample Chlorine often, and or for extended periods. Based on what you say, it shouldn't be, but more info is needed for us to help determine that.
Lastly, if you've had BA in the past, it is extremely likely you never got the roots and killed it all. This happnes a lot, so has that ever been the case?