I think though, it is implied in Immortal Blood that the Volkihar can't blend in with the living.
" I told him about the most powerful tribe, the Volkihar clan, paranoid and cruel, whose very breath could freeze their victims' blood in the veins. I explained to him how they lived beneath the ice of remote and haunted lakes, never venturing into the world of men except to feed."
I think Harkon's warlord background gives rise to the castle they dwell in...
But are they splinters of the Volkihar, or are all the vampires in Skyrim some manner of Volkihar breed, born of Molag Bal or infected by the Volkihar themselves?
Too much is up for debate to say, I think.
But they don't blend not because they are ugly they look rather human they don't try and do it because they don't care for human society as you see in dawnguard they only start caring when people start messing with their food and then they simply go out and curbstomp the problem and go back to the castle. They are like gangrel from the world of darkness.
As it said in immortal blood the volkihar are
"paranoid and cruel" and harkon's court pretty much fit the bill in terms of their quest line and side quests heck they eat people off the tables.
Heck the vampire faction see the dawnguard faction as a side quest their whole main quest line has nothing to do with em either. Lets not forget when it finally comes to killing them its more an "Yeah these guys are getting kinda annoying take care of them will you?"
Oh and harkon says that they are the oldest vampires/clan in skyrim so they are volkihar plus the order does not have a presence in skyrim I think they pretty much like having control of cyrodil since it fits their taste and they simply destroy any other vampire clan that tries to enter cyrodil.
Bethesda captured the volkihar personality really well imo when it releases on the PC I will give myself the ice breath shout and ethereal and specilaze in ice magic I will just call the shouts vampire abilities/magic.