The thing is, it's impossible for a saber to simultaneously "look" bright and not illuminate a room. The light that is shining on the walls and other surroundings is the same light that's shining on your eyes. If the blade were not allowing any light to escape, you wouldn't be able to see any glow from it at all!
Also, it's not true that a lightsaber blade does not illuminate what's around it. For proof, check out the duel in the dark between Anakin and Dooku, or Luke's vision in the evil cave on Dagobah.