Say they had pale skin, what would they look like? Would the lips be black? Would the vascular system show through the skin more? I'm not very interested in how or why their blood is black, just how it would effect their appearance overall.
In cold conditions and when sedentary these people would appear corpse like with very pale skin is if they had pallor mortis
In warm conditions or after vigorous exercise these people would appear in shades of grey as if they were a living black and white photo, although given their coloured surroundings a lot more creepy. They would not look exactly like that due to other factors such as eye colour, hairy colour, other elements in the skin and clothing, but it would be a good approximation especialy if naked.
Some thoughts here...
Consider when someone blushes -- the red is pretty obvious.
Now consider when someone is about to faint ("goes white" they say). This condition is often described as someone turning ashen.
So clearly the blood is a factor. I can imagine people looking more or less as per standard, though with a bit of a goth look ... lips might be more or less dark gray, and the skin would be pale with the dark underneath, might look "waxy", because the top layers of the skin are a bit translucent. Their tongues would be black, too.
On these guys, "bloodshot" eyes for heavy drinkers would look creepy and awful.
Side note: there'd be no emo or goth phenomenon, because, well, what's the point? Their parents have it covered...