You should be able to get jet black using manganese without much trouble.

Over time the free acid goes up and the result is a lighter coating. If nothing is done, the free acid will eventually increase to the point where it consumes the coating and nothing is deposited on the surface. The surface will look gray, but it’s just acid etched.

To get it to work again you need to bring the free acid level down. Adding steel wool or scrap steel will use up the acid. The problem with doing that is you end up with a lot of iron and it ends up in the phosphate coating. It looks black as well digger’s a___ , but it is going to rust. And when you think about it you have been putting parts in the tank….. adding iron. So there should be plenty of iron.

You can test a small amount of your solution by putting it in a glass and heating it in the microwave. Then add a little disodium phosphate to bring the acid level down. Dip a test part in it. Add a little more disodium phosphate if needed. It shouldn’t take much in a glass. If you get a good black result, add disodium phosphate to your tank. If you don’t get an improvement, try adding acid. It’s unusual for the acid to be too low but it can happen.