Between 2009 and 2016, the apartment used for Mick's residence appeared in Castle (2009) as the loft apartment of Richard Castle, the show's lead character.
As a vampire, Mick St. John regenerates and heals all wounds. However, throughout the show, Alex O'Loughlin clearly has his left ear pierced. The holes, to a vampire, would have healed.
I think that you will find that each of the glowing letters in the credits reference different blood types, with O being the most common.