The UESP is the best site for gameplay information, but not the best for lore.

Well that explains it then
I was originally under that same impression, that they are simply inhabiting Volkihar Castle, and are not necessarily Volkihar themselves. I do remember them only mentioning the name of the castle and that they've occupied it for some time. It's entirely possible that the real Volkihar are dormant or even extinct and Harkon just moved into an abandoned castle.

Oooo! That would make for some great DLC for later. RISE OF THE VOLKIHAR!

Only problem with this is there are NPC Volkihar vamps.

Well that explains it then
I was originally under that same impression, that they are simply inhabiting Volkihar Castle, and are not necessarily Volkihar themselves. I do remember them only mentioning the name of the castle and that they've occupied it for some time. It's entirely possible that the real Volkihar are dormant or even extinct and Harkon just moved into an abandoned castle.

Oooo! That would make for some great DLC for later. RISE OF THE VOLKIHAR!

The Elder Scrolls universe would not be affected by the sun going out the same way Earth would be. In fact, if you finish the quest with the crimson nirnroot, you will get a book that actually says the sun went out for a year once. All that happened was nirnroot started disappearing. All the other plants "found a way" to survive. The sun is just a hole ripped into the realm of Oblivion from a god leaving.

You can read up some of the lore about http://www.uesp.net/wiki/Lore:Nirn and http://www.uesp.net/wiki/Lore:Mundus on those links. Also, http://www.uesp.net/wiki/Skyrim:The_Nirnoot_Missive I mentioned.

So basically, the sun has already disappeared for a full year, all it did was remove a bit of magic from the world.