The best you can do for her at this moment in time is to get/keep water at 0 ammonia, 0 nitrite and <20 nitrates (or no more than 20 above tap water nitrate level). Black is often a sign of ammonia burns.
I'd also ensure that you feed some veg, just in case there's a blockage internally that accounts for the floatiness. Take the shell off a pea, cut each half into tiny morsels, and feed that.
She possibly looks slightly bloated on one side - this doesn't necessarily lead on to anything but I'd monitor her closely. Do you, by any chance, have eSHa 2000 or Waterlife Myxazin to hand? If the situation deteriorates, especially if her scales stand up like a pinecone, then it may be worth treating with that. Melafix is fine (I've used it successfully) but, with labyrinth fish such as gourami, you need to be all the more careful *not* to overdose - err on the side of underdosing if at all possible.