The all-black members are the "Grim Bastards" out of Lodi. They aren't Sons, but they are a long-time associated club that may be considered a patch-over in the same way that the Calaveras are a patch-over for the Mayans.
The only indication that we've been given that the Sons don't have black members was the Sheriff's comment when he wrecked the wall of photos. Beyond that, there has been absolutely no on-screen evidence whatsoever that SAMCRO is a WASPs-only organization. Hell, we also know that Bobby is Jewish. Sheriff Roosevelt, or whatever his name is, makes SAMCRO look like the Klan on war-training.
All in all, the whole race card thing just seems to be an anorexically thin plot device to manufacture false tension within the club. I'm hoping this will all be explained away at some point, but it's not looking like it. Other than this, though, I am thoroughly enjoying this season.