Have not bought it yet but want a dark blood-covered home for my vampire lord? Any chances i can do that?

well the celler can have a coffin in the wall if your already a vampire.

otherwise the house really isn't dark unless you opt out of making lights.

There are no “theme” choices, if that’s what you mean. Your dark mood would have to come from your own decorating. Bloody rags, bones, and the like from your inventory.

So i can place things without them getting moved around when i leave the room (as with normal skyrim homes)??

Things will randomly move around as always (maybe a little less, don't know for sure). I find my necklaces and rings lying on the floor everytime i return home.

And sorry, you can't adopt either Babette or Aventus.