There were several people in the club on the outside and somehow, because of how secretive the interaction between Blair and the strange figure was, they failed to even detect anything strange about the duo's movement.
Blair had opted to follow the strange figure's aura which led him away from his initial target. He had come to the club with the hopes of finding this strange demon he had once fought with on the night of Klaus's evolution. But on getting into the club, he knew there were a lot more to do than just tracking down one demon.
He and Erwin had every reason to believe that the club was an epicenter of human feeding and that the demons that ran the place did it so well that no one ever suspected a thing. It was easy to pass off as humans as there wasn't any outright difference between both race if one took physical appearance into consideration. Detection was fairly not an easy job in the case of humans.
