The phrase blue-shirted guy refers to an unnamed background character seen throughout the series who wears a blue t-shirt with a folded-down neck collar on it. He has light brown hair and wears glasses.
"A flying Skateboard!" -Mr Big's Big Plan. Give me a number
Yeah, the let--
He's seated on the far right part of this snapshot of the audience.
He is seen as a victim of mind control in one episode.