Since we do our paraffin-section DAB 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 reaction increases the contrast quite a bit.