Quote from Marshall in The Pineapple Incident
Ted: We're not really doing shots, are we? Lily: I hope not. Barney: No, no. Lily: These look kinda like blood. Marshall: Okay, I know that you've all dismissed this theory before, but is there any chance that Carl is a vampire? Barney: That's ridiculous. Marshall: I'm serious. Think about it. He always wears black, we never see him in the daylight, only after dark. Robin: Oh my God, that does describe a vampire. Or you know, a bartender.