Blood dragons are a faction (bloodline of Abhorash) of vampires, the knights of blood keep used to be an order garding the border to Bretonnia. They are now called blood knights after being tested by Shaitaan, so are now part of the blood dragons. That is why it is a bit silly for them to be in a von carstein book, since blood lines fight each other.
The 7th and 8th edition army book will have almost the same information on the blood knights, like almost none. There is more to be found on blood dragons but they are and were not an empire order.