Sorry about the confusion about Liam/Angel. I just used it as a way to differentiate between Angel as a vampire and a human. I'm well aware of the fact that Angel wouldn't revert back to his historic persona if he became human again.
As for the black blood. In Issue 1 (coloured by Ilaria Traversi) the demons Angel fights in the beginning have red blood, as do Kr'ph's henchmen. Also, Angel has a red blood stain on his shirt, when he walks into W&H; it would seem that it's his blood, as on the same location he applies the healing-creature. On the final page with Gunn, his (human) female victim has red blood too. In issue 2 (coloured by Jason Jensen) the blood fountain is red too (although, we don't know the origin of the blood). On the last few pages when Spike and Angel exchange fist, you'll notice that both Angel and Spike bleed black blood. The third issue is again coloured by Jason Jensen. Beside Angel's blood, we also see some blood on the face of the (presumably human) security guard (which is black too) and when Illyria is spit out by the dragon she is dripping with black blood too. Although, she's just been thrown through the fountain, so we don't know whose blood that is.
To be honest, I really don't know what it all means. I'm going to go with it being a style-choice. But if there is some kind of a meaning to it, I'm sure as Hell(.A.) not getting it.