IMO, you'll be fine. A good judge is looking for the best horse & rider, not judging the color of your tack.
That said, you already know you will likely be in the minority by not using light oil tack, so spend extra time putting together an outfit which really complements you and your horse because you will stand out a bit more. Make sure your bridle & reins match your saddle as closely as possible - that goes for leather color and silver pattern & type (silver plate doesn't shine the same as higher quality silver, but that's more noticeable under arena lights than outside, IMO.)
I saw a few black saddles on black based horses at Paint World in the last few weeks, and there were a few dark oil ones in trail. Western Pleasure was nearly exclusively light oil, of course.
Last edited by Cynical25; 11-19-2013 at 03:00 PM.