Since we do our immunostaining in an EM lab, we often use
osmium to darken the reaction product.  We simply put a
drop of our standard 1% cacodylate-buffered osmium tetroxide
on each section in a hood and rinse it off in running tap water
after about 2 seconds.  It makes light DAB reaction product
pretty dark brown, and adds a light brownish tint to the rest
of the section.  The hood is crucial, though -- without good
ventilation, you will start to notice rings around lights very quickly!

Hal Hawkins
UTMB Galveston