I've battled this too. For my passive intakes I've used the $25.00 Doran 8"x8" light proof vents. - look at B&H Photo: Darkroom Fans & Louvers | B&H Photo Video Work great.

Great deal here:Doran Dark Room No-light Fan and 2 no-light vents - eBay (item 290229610744 end time May-13-08 11:25:37 PDT)

A more ghetto solution is black pantyhose. I used it on my passive intake for the hood vent. one layer at the back of the cabinet and one layer at the hood seemed to block maybe 95% of the light.