Oh Jesus my commenters above, whom I love, the Black Party is a circuit party that is the "kink" party. Think leather, WS, FF, gangbangs, and anything else "extreme." Now, most people who attend "black parties" are more vanilla and simply enjoy the scene for a night, but there are plenty of hardcore parties who engage in systematic sex. No judgment, it's awesome if you want it.
I've attended two black parties (could have attended more but passed) and loved both. It's really fun.
OP, wear whatever you want. It's not like the white party, where it's a 'thing' to wear white. The black party is a tongue-in-cheek reference to kinky and dungeon-esque things, not the actual color black. Though you can't go wrong at the black party in black--and to be fair you can't go wrong wearing black to any night out in gay world. But we meander.....
Go, have fun, wear ANYTHING you want. If you're in Chicago, a major city, I'd say the following after years in the clubs around the US: (1) wear close-toed shoes, no flip flops or sandals, (2) wear a shirt with a collar, but also an undershirt that is a "wife beater" or a t-shirt of some sort so you can relax once you're in the club/bar, (3) trust nobody to get you a drink and drink nothing handed to you by anyone other than a bartender, (4) keep your wallet in the front pocket, yes it makes you feel paranoid, but if another one of my friends who didn't follow my advice says "you were right" I'll freaking slap somebody, (5) take case with you, at these venues it gets "overpacked" and the "mac" machine will say 'out of funds' or 'out of order' and you'll be at a bar taking only cash, no cards, and bumming cash from your friend. Just take $200 in cash or however much you drink (obviously more if you "pop bottles").
Most of all, have fun. It's a really cool circuit crowd, my favorite.