Don't mean to be useless but here I go: Why? On ring, even on silent, the whole screen lights, vibration vibrates (if on), kbd lights if visible, how would a blue LED help anything?

And, IMO your best bet is to use PhoneControl script (it has flash led <pattern> and call it from any program that does stuff on ring - espeakcaller, recaller, etc. Pick one in Python, it's easy to insert a line in there somewhere.