Mystery surrounds the black bean soup at Cafe Miami (6114 Bissonnet, 772-3042). One person I talked to there said it contained garlic and onions. But another disagreed. What made the soup special, he insisted, was cilantro and bay leaves. I myself thought I tasted sherry and lime juice. Who knows? Maybe we were all right. Or maybe all of us were wrong. Not that it matters. Whatever's in it, the soup is quite, quite wonderful: bracing and nuanced and very aromatic. It's cheap, too, a cup costing a mere $1.25. If only all our pleasures were this inexpensive.