Blaine has only talked to her properly once, I think? He chased her through the woods once playing flirty tag not knowing who she was ahahah. He gets the feeling that she's dangerous to him not just because she's a vampire, but because she loves Matthew so much. She would give Matthew anything and here comes Blaine, sweeping Matt away even if it's only for a blink of an eye in their lifetimes. So he's wary of her, but appreciative of her knowing that she'll take care of Matt if anything happens.

But he still thinks she might shank him if she could, despite how polite she is.