Black is the hardest to get right with a home setup. A few questions.
What grade of aluminum are you using?
Are any of the parts touching the cathode?
What material is the cathode?
Are you rinsing the parts in clean water between each process?
What type of dye are you using?
Are you agitating the cleaner, desmut, dye and sealer tanks?
What temps are the cleaner, desmut and dye tanks?
Nickle Acetate sealer works better than boiling water.
I get better results with black if the acid solution is 10% at 65F or below. I freeze Ziploc baggies filled with water and drop in the acid tank.