There is no need to use a dark slide if you are not changing out backs in mid-roll. You can use it to keep dust out of a loaded back off the camera before or after the roll is shot. Inserting/removing the dark slide should not cause a leak, but it can if the light seal is worn or damaged. Bronica recommends not carrying the camera with the dark slide in place because it defeats the interlock and the back can come off if the release button is accidentally depressed. Contact Jon Goodman (who is here on APUG) for one of his excellent light seal kits. In the meantime, don't use the dark slide and tape over the dark slide slot for extra security. You can change film by removing just the the insert to reload.
As to the blotches, clean the insert thoroughly with a alcohol on a Q-tip or light cloth like a microfiber cloth, and make sure to blow out any fibers afterward. That might help.