I'd say jumping in a black saddle - after all, it only matters how well your horse jumps, not how you look. And most the time, you're too far from the judges for them to notice anyhow. ALthough this depends on the cousre. Why do you ask?
It probably depends on the level of the show. People at the local hunter shows around here would look down their noses at a black saddle, the rated shows would be horrified! I've been to some schooling dressage shows in my brown close contact saddle, completely ignored the strange looks and won! My friend even used her brown close contact saddle at a rated dressage show and won as well! If you make sure everything else is clean and trim (pulled mane, clean and shiny coat, proper clothing, nice tack, etc) it would downplay the color of the saddle. If you're trying to decide what color to go with I'd pick whatever color you would be more likely to compete in the most.
Thanks for the replies everyone. I'm looking at a saddle I want to buy and I can get it in brown or black but I want to do both jumping and dressage! Lol. It seems like the vote is tied......I was told by a friend who shows Arabs that dressage in a brown saddle is more acceptable. I ask because I want to show in the future but need to get the right color......darn the choices! :)