The best thing to get a black tire, is the surface prep. Though, it's a bit of a pickle, because you really gotta scrub the **** out of the tire surface, but then, if you use something too harsh, you'll take the anti UV crap off of the tire, making it age/crack/rot WAY quicker than normal. If you don't drive your car much, and it lives in a garage, get a good brush, and some wesleys bleach white. Or just some really hot water & good dishwashing detergent, and clean the hell out of them. Wipe them down with a BLACK lint free towel (good luck with that) or just let them air dry in the most dust free enviroment you can. Then use whatever tire dressing you are experimenting with. Several washes may be necessary to get all the gunked on brown stuff completely off.
It's all up to experimentation. And man have I done a lot of that lol.