Though generally speaking dragon blood is depicted as being just as dense as regular blood, if not moreso, and possibly resistant to coagulation, way moreso than the Blood of St. Januarius (as in, staying a mobile liquid all the time, if kept in a closed container.)
Blood makes up about 10% of an animal's weight, and one of the things we need unobtainium for is to drastically reduce the dragon's density.
The scale I described above was a 40-foot body, 200-foot wingspan. If you look at Drogon in "Game of Thrones," he seems about that size or a bit bigger (his season 8 length is given as 150 feet, but only about one-third of this is body, with neck and tail making up the rest). At this scale, you need the dragon to be 20% of the weight of a normal animal, so you
could achieve the necessary lightness without altering the blood.
To go much beyond that, however -- if you want dragons on the scale of Vhagar or Smaug -- the blood
has to change. Either it must be less dense, or the dragon must have a lot less of it. Either way reduces the amount of work required to pump it.
I was assuming we would simply be reducing density across the board, so everything including blood is 20% (or 10%, or less) of normal weight.