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.
The only type of showing you can get away with black tack on any horse is gaited shows (and most still prefer brown), Dressage, and timed events (like barrel, jumpers, etc.). If you show up in black tack in almost any other event, you will be marked down, or not even placed (depending on the event or judge).
Traditional horse tack for pleasure or show is some shade of brown. Black horses look good in a more chocolate shade of dark brown, western = dark oil and in English = havana. If the black horse has some red tint to it, or fades a bit to a reddish brown, then a medium oil or chestnut colored leather can look good, as long as it's on the darker side.
I do not like this, though it's what you have to do if you show breed show WP...
This is better, more medium oil tack than light oil as above.
I think pretty much all tack looks nice on black horses as long as everything matches, except for really light caramel coloured leather. I'm not a fan of that colour on any horse, except maybe on some palominos.