If discus turn to black maby have external parasites (gyrodactilus, dactylogyrus). Please read on this forum something about salt bath or something with formalin, praziquantel... and try. But if discus stop eating have internal parasites like Hexamita and company. In this case you must turn the temperature in your tank of 93-95F and left discus in this temperature 15 days. In two or three days discus must start to eat. If no, you must ad Metronidazole to ( 600mg/100liters) and have discus in this concentration of 3-4 days than wc 50%. You must repeat this treatment with 14 day. You have many discusion about metronidazol on this forum. Take one of this.
I have one question of about this to. One of my discus have black only one half of head from nose to end of the gill. Have any idea whats this? Discus eat and swim and breath normaly. Second side of head is normal color. Only this black color of his half of face.
My English is not good and I hope that youll understand me.