Natalia often uses her desirability to win fights in training combat against Vincent. In this scene, she is fighting another man, and is trying the same tactic. You’ll have to read the book to find out how this turns out.
From Inside the Grey House:
She was within arm’s length of Joseph now, her hands behind her. She stopped and very carefully cut her thumb on the rapier. She let the pain show on her face, even if it didn’t really hurt. Sure of herself and of the vampire’s hungry look she brought her injured hand to his face. She placed her bleeding thumb on his lips, letting the other fingers rest near his jaw line. His eyes closed as she traced his flesh, leaving blood lipstick on his lips. She pulled her hand away.
“Lick your lips clean, vampire. Sample the consolation prize.”