Actually it should be that way. According to lore Warlocks mastered blink, and tried to keep it a secret since it was taboo in the city. Well crafty hunters saw it and stole it. Hunters abilities according to lore are never really taught but rather unwilling gifts from others. So to answer your question why do Warlocks have an exotic that enhances blink. Lore wise, its because Warlocks have a vested interest in blink when a hunter is more likely to just steal their findings. Gameplay wise, its because BladeDancers have the quick draw perk and if you add that exotic on to quickdraw it would make all the updates to Blink null