His dick is so dark as a result of "clamping" and maybe also pumping. Over at Thunder's place.org, it was explained how the dark colour is a side effect of clamping regularly for several months. It is also the most effective way of increasing girth (WARNING - for advanced PEers only!!!!)

If you listen closely while he unsuccessfully tries to fuck her, you can hear the water in her fake boobs swilling about, hilarious!