White balance will shift the color of the image, not the exposure. See page 60 of your 7D manual for white balance settings.
If you're consistently underexposed by one stop, does it occur with a specific lens? Do you have a Minolta 50mm 1:1.7 lens that you can also test with? That one should produce correct exposures. Sigma lenses have a chip in them that may need adjustment to get correct exposure.