I've always found this amusing. There was no hostility between Kael'thas, Tyrande, and Mal'furion when they met in Warcraft 3. In fact, it was Tyrande who offered to aid Kael'thas and his people. Also, if it weren't for Kael'thas, Malfurion would have continued to believe Maiev and never learned that Tyrande was still alive and considering the situation Tyrande would have eventually fell.
It is to my understanding that Malfurion and Tyrande did not know who/what the Blood Elves were because they were so different looking. Same reason that the Naga were unknown.
As well, the act of one does not make up for the racial view of the whole race. Remember, these are the descendants of the same outcasts that nearly destroyed Ashenvale with a magical storm.
We are talking about spiritual leaders here, the examples of a race. They were well aware of what they were, in fact they greeted each other appropriately and this little piece of the conversation.
"Maiev: Kael, where are your high elven brethren? Do they not fight the undead
as you do?
Kael'thas: The Scourge devoured our ancient homeland of Quel'Thalas. The once
proud bloodline of my people is nearly spent. The few of us that remain now
call ourselves blood elves, in homage to our murdered people.
Tyrande: I grieve for your people, Kael. But you must not allow rage and
despair to poison your heart. You may lead your people to a brighter future."
Oh, and prior his corruption by the Naga, Illidan, and the Legion, Kael'thas was a cool dude. Unfortunately, Maiev was always a %@!@*.
"Kael'thas: We must hurry to save her! That current will take her straight into
the heart of the undead lands!
Maiev: No, Kael. Tyrande is a soldier; she knew the risks she took. We have a
greater mission to accomplish now, and our time grows short. Your people are
now safe. You will uphold your end of the bargain and help me hunt the demon I
seek."