I see that a blue light is legal on an authorized vehicle: "(3) Blue lights. A vehicle may display blue lights as provided for by 75 Pa.C.S. § 4572(a) (relating to visual signals on authorized vehicles). Vehicles described in 75 Pa.C.S. § 4572(a) may be equipped with a light-bar assembly using only blue lights." but I don't see where just any vehicle can display a blue light, flashing or not.

Just a side note--I'm not trying to give you grief for the hell of it, just don't want to see a fellow cyclist hassled by the local police.