It is a bit more complicated, some of the blood elves didn't like them consuming other living creatures and fel energy to sustain their desires. Generally the measurement used to difference the two is the color of their eyes (this is proven by Iridi in Night of the Dragon) in other words whether they have or have not consumed fel energy. Wjat we learn of the blood elf death knight in their starting zone though is that he/she died defending Quel'Thalas against the scourge. In other words he/she would already have been a death knight by the time the sunwell was destroyed in other words he/she would never have lived to get the opportunity to consume fel energy. Not to mention their eyes are blue.
There are of course other measurements regarding the difference between sin'dorei and quel'dorei I'm just using the most widely used one.