I get this occasionally, and find building cubemaps / restarting TF2 tends to fix it:
To build your cubemaps, open the console with TF2 open and fire in the following commands one after the after:
-mat_specular 0
-map <mapname>(Load your map)
-buildcubemaps
-mat_specular 1
-disconnect
-map <mapname>(Load a different level; clears texture cache)
-map <mapname>(Load you map again)
If its still in full bright after reloading your map, try restarting TF2 and loading the map again.