That black patch looks much darker than the bbw. A pool of manganese?
Yes, I think there could be manganese mixed up with BBW. The BBW looks almost black at the last photos, I think this is due to the lighting (halogen) and some dirt or dust on the stone. In daylight, the back of the stone looks more like a BBW than a Dragon´s Tongue , but is still very dark. The black patch is really black, but in sunlight, it has a bit metallic blue in there.That black patch looks much darker than the bbw. A pool of manganese?