I would think that outsiders could be a fairly diverse lot so the magic could potentially look however you like. Outsiders are supposed to be kind of Cthulian beings so perhaps look to Lovecraft for some inspiration.
Agreed, go with whatever feels right to your campaign. The only examples in the books are The one who walks behind & his entropy curse, the superghouls themselves, and the outsider fueled despair that ummm... malvora pushes with the suerghoul attack. The mordite is supposed to be from the outer realms (where outsiders come from).
. To steal some d&d and dctor who terms, picture an alternate plane that mixes xoriat, pandemonium, & the plane of negative energy with nasties for inhbaitants that aim the dalek "exterminate!" philosophy on the other planes