rokket, bagginsbill mentioned that current poser version(s) calculate reflection wrong using reflection channel, and instead recommended using alt_diffuse with reflection and blinn nodes.  I noticed your mirror darkens scene.  as I can't duplicate your scene and render with fresnel procedural, I can only suggest increasing raytracing/IDL quality/bounces and using GC, with view to less darkening, pending release of further info in re: reflection settings.

I recall the mirror-darkening issue arose earlier, and one of them may have addressed it already for the case of reflection channel with no GC.  like a glowing mirror or one with non-zero diffuse (both fake).