Any visible dust is not on the mirror, but on the viewing screen
or on the prism surface above (which is why a blower should NOT
be used the on mirror or viewing screen - blowing will result in
dust being forced up over the viewing screen where it can only
be removed by a repairman, unless the camera has interchangeable
screens [with the screen removed, I would blow off the lower
surface of the prism]). Minor dust on the mirror has no bad
effect, but a finger print does. Breathing up onto the mirror
gently just before LIGHTLY wiping with a CLEAN cotton swab will
remove most prints, but at the risk of adding a few light
scratches to the mirror (optically meaningless, as long as there
are only a few light ones). Hope this helps.