Black coloring on Catt bulbs

[Royal]

Black plant tissue is usually bad. BUT, it seems from the picture that you may be talking about the papery sheath around the pseudobulb. The outermost leaf will sometimes become this papery covering that eventually flakes off. It can cause issues if it is allowed to trap water, but it's normal for many plants with this type of growth habit.

If the actual pseudobulbs are black and mushy, I would probably take it out of its pot and remove all of the diseased tissue, making sure that each cut (with a sterile tool) is made into healthy tissue. It's a drastic approach, but clean margins will ensure you got all the bad stuff.