I agree, sounds like Black Ich (look up: Turbellarian - Paravortex for more info on the bug). If this is the case, which would be evidenced by small discreet black spots, you should be able to clear it up with FW or Formalin dips. There's an outside change the fish is affected with intermediate stages of digenean trematodes (which also causes a form of hypermelanization). Hard to tell without a clear picture and more info. I would suggest trying FW and/or Formalin dips and see if this improves the situation first before considering other treatment options.