I think it's normal. I never noticed it before your post but mine does something similar. With the 5DIII and a f/2.8 lens mounted, the point turns grayish with I point it toward a strong light source. The effect is stronger when I tried a f/1.2 lens. If I don't point it towards a strong light source, it is darker. Either way, I have no problems seeing the AF point, unless trying to AF under very dark conditions. Probably has to do with the large aperture and amount of oblique light getting around the LCD layer to make it less black, so I don't think it is something that can be improved easily. It never bothered me before and won't bother me in the future.