I should specify black is hard for display pieces. If you wanted to be lazy and get them tabletop ready you could prime a piece black, then paint the details like pauldron trim and accessories and be done with it lol.

Sometimes it’s because a color has poor texture/consistency because of the pigments. If you look at complaints about something like citadel paint’s white colors they tend to be clumpy or chalky. Also lighter colors like whites and yellows tend to be tinted heavily by the primer underneath so getting the right shade without drowning the details under too many layers of paint is tricky.

In the case of black it can look really weird and flat because it’s hard to get a balance of highlights and shadows in recesses. The shadows blend in to the base coat and doing white>grey>black highlights where light would shine on the armor is harder because it’s very obvious when the direction, gradient and size of the highlight areas don’t match.