the SCOBY (the 'mushroom')can go through through a lot of variations--as it's getting established in its new home.
I'm guessing when you say 'almost black', you mean a kind of slimy darkish-looking substance. Like a very thin scoby, trying to form. I've had that, changing over from a scoby grown on black tea to my preferred green and sometimes green and white mix. It might depend on the kind of Sugar too.
As long as it smells good, and the stuff on top is not moldy--that is, back and fuzzy, or very evil-looking, it's probably just fine.