Basically its up to you and your goals/preferences. I see lots of blacks in brown tack.
If you show hunters, brown tack is preferred in the ring. Dressage classes go black, jumpers I don't think it matters that much. Western, it's up to you. My grandmother's black QH looks pretty good in his black and silver tack and a black/cream/silver pad, but I see lots of blacks in light oil or brown tack.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown