Never used a blue flag message, never will, it's pointless as the Driver knows It already.
I have been on the receiving end of blue flag messages in the past and they pissed me of big time, I think they can produce the opposite of the desired effect.
Saying that I do think there is a problem with people pitting out mid race and not understanding there NOT IN THE RACE and therefore should not impede or slow down the people who are in the race , all I do know is screaming BLUE FLAG at them is counterproductive.