Here is how to reveal a blue in fel, if the hidden char is damaged then there is a chance to reveal you can damage a hidden char using the following:
conflagration potion, explosion potion, thunderstorm, poison field, meteor swarm, hail storm, chain lightning, earthquake, (hit area on a weapon if you hit something nearby), poison strike (you'll have to cast it on yourself stood within one tile of the player)
I think that is all. Please note you must be red to damage a blue with these unless the blue has flagged on you.