Sorry she isnt feeling well

If you posted a picture i'm sure some of us could help you know what's wrong. I will do my best to help you in any way i can.
a couple questions:
Did you fast her? if so how long?
How big is she/he?
How long have you had her for?
What do you feed her and how much?
Any sudden changes in water quality recently(like mini cycles,
pH changes, etc)
What does the growth look like (the black one)?
How big is it?
Is her belly sunken inward?
Is she pooing?
Any physical damage to her wen, gills, face, etc? (sometimes they can get it because they drag along the gravel)
some of these questions may help us know and guide you. I am really sorry she isnt feeling well.
Best wishes.