The wikipedia article didn't clear things up for me! (Of course, I could have just missed the important stuff) The article states Arneson gave up all rights to D&D to WotC . . . but doesn't make clear whether that included the Blackmoor campaign setting. The article then goes on to detail many of Dave's projects since that point, many of which dealt with Blackmoor. Did Dave license back Blackmoor from WotC, or have some sort of agreement with them?
Recent events suggest that's the case, but the curious . . . well, are curious!