It's not so hard to find them now. But nearly 10 years ago, when they appeared in " Medicine for Melancholy," the first film by the "Moonlight" director Barry Jenkins, characters like Micah and Jo' - young, black, financially overdrawn but rolling in polished pop culture references - were, if not exactly unicorns, a protected species, rare enough to be worthy of tapping the person next to you and spreading the word.