Richard is the protagonist, because, well, the story is about him. Richard really is the only candidate for a protagonist, since he’s the only fully developed character and the only one who has any character growth. He’s also the narrator, and Black Boy is about his emotions, his life, his hopes, and his dreams. So yeah, we’re totally certain that he’s the protagonist.
All you need to do, really, is check out the opening sentence: "One winter morning in the long-ago, four-year-old days of my life I found myself standing before a fireplace, warming my hands over a mound of glowing coals" (1.1.1). Me, my, mine. We get it, Dick.