Mrs. Blake
Damon glanced at Talia to confirm that she didn't react when the man called him 'Mr. Blake', and Damon took that as her approval. Yup, she definitely didn't mind using his last name.
There was a time when werewolves didn't have last names. Even in modern times, they identify themselves by the first name, pack they belong to, and their position.
However, since they are coexisting with humans, each werewolf family has a surname. It helps them blend in.
Of course, prominent werewolf families are an exception, because their last name also signifies status, and the surname Blake is like royalty in the society of werewolves from the time before Damon's grand-grandfather.