I wonder if Bethesda patched that in 1.5, then... because I've noticed lately that dungeons seems much darker than they were just a couple weeks ago, to the point I'm considering increasing the brightness settings.

It's Ambient Occlusion (SSAO or just AO), makes everything dark quite dark, enhancing the contrast.
Disabling it will lighten up the entire scene.
Also, your lighting distance (how far any/all lights in the game extend) is adjustable from the advanced graphics tab.
Whacking the slider to Full makes an enormous difference to the visibility in my game...
Bit more work for your vidcard though.