I've heard of several ways to clean filters > i.e soaking filter in petrol to remove junk, let it dry completely, then treat it with the air filter oil, re fit to car. <-- dont think id try it, though it cant hurt - only a filter after all.
And the obvious other method is buying the cleaning kits. Maybe try searching the forums, and even google, youre bound to find an answer somewhere about the Blitz filter.
I have the apexi pod filter that apparently doesent require maintenance - so ive never cleaned it - except for removing dust from the grooves with a compressed air gun.
As for the black stuff spurting out the rear > is your car running a cat convertor?
You'll generally find that exhaust soot will land itself on the rear of your car if the tip of your exhaust ends pretty close under your rear bumper. Mine sticks out quite a bit, so i never get soot on the bumper.
It could be just caused by backfiring. Generally aint a problem mechanically.