I've seen several folks using multi-color LEDs -- including blue -- on their wheels and frames. Haven't seen anyone using blue lights that might be mistaken for emergency vehicles.

Not sure it's even necessary. I've noticed cars yielding to me, even pulling over to the right and stopping, and realized they mistook my flashing white and red lights for an emergency vehicle. And I'm just using fairly ordinary lights, nothing particularly bright. Reminds me that for every driver who fails to show ordinary courtesy, there's at least one uncommonly conscientious driver who memorized every civics lesson.