[ her lips parts some realizing he's going to boldly drink some, here and now, and she follows the movement - but not with any disgust. it's mere intrigue and surprise, and both are adjusted in the following moment. elena's seen damon and stefan and caroline drinking blood multiple times. truthfully, it's still weird for her sometimes (her best friend since grade school drinking blood might throw anyone even after a few months) but she's used to it, and the weirdness, by now.
him drinking does give her a moment realize something. ]
You had to know what you were dealing with in those experiments.