Later, during the Third War, the Alliance extended membership to the night elves, who saw it as their duty to warn their new allies against the high elves. The Highborne's part in bringing about the War of the Ancients figured heavily in the night elves' retelling of elven history. It should come as no surprise that humans came to regard the high elves (and, by association, the blood elves) with resentment and suspicion.
[...]
Certainly the Alliance has shown no interest in helping the Azerothian blood elves either reach Outland or drive the Scourge out of Quel'Thalas. Already suspicious of the few scattered high elves who still exist, the Alliance considers the blood elves even less trustworthy. Night elves are particularly hostile toward blood elves: to night elves, the sin'dorei stink of desperation and arcane magic. (
WC Encyclopedia)