Artist's charcoal would probably work for what you want, they come in stick form (similar to chalk) and and a more natural form (like a burnt stick) with varying degrees of hardness. You can find them at any decent art store like Aaron Bros or Michael's, just look in the paint/artist supplies section and you'll find them there. After you're done applying the charcoal just be sure to apply something like Dull Coat or a clear coat to seal the charcoal otherwise it will just simply brush right off.
If you already have your flightsuit this would be an excellent opportunity to weather that a little too by wearing while doing the blaster flashes on your armor. Working with charcoal is a messy business and charcoal dust would make for an excellent weathering agent for the flightsuit although I think it might wash out of most clothing; it's been a long while since I've last used charcoal so I don't remember how well it stains in the long term.
