A black rectangle with a solid, thick blue line through the middle. Symbol that is put on items ranging from front license plates, pins, bumper stickers (very small sticker),etc that denotes the user is a police officer. It is a violation of many state's laws to display this symbol if the user is not employed by a police agency. Related to this is the Red Line, which is used to identify fire/medical services personnel. These symbols are usually found on the off-duty vehicles or other personal effects of police /emergency services personnel.
My next-door neighbor, who is a highway patrolman, has a Blue Line tag on the front of his car, as well as a Blue Line sticker.
According to Canadian-born Robin on HIMYM, going to the blue line means just kissing. Otherwise known as first base in American culture.
"It can't be his baby, we just went to the blue line!"