Pull the dark slides out, open the flaps and vacume the heck out of them. Use the micro attachments on a shop vac or something similar. Shoot them with compressed air at a gas station, bring them home an vacume them again Wipe them down with a clearer or better yet, glass wax them clean. If they are the Graflex kind with the traps that can be unscrewed, open up he traps and vacume the velvets. Unless you know the slides themselves are made from aluminium or similar, avoid using water. Some dark slides are adversely affested by water.