Just gonna throw this out here. AeroTech Consumer Aerospace makes a model rocket motor propellant (solid) called Black Jack and Black Max that leaves a heavy black trail on ignition. It might be worth dropping them an e-mail and seeing if they'd reveal what kinds of compounds burn black, as I'm sure that they don't worry about an incomplete combustion process. Possible you could liquify something out of what they tell you and pump that into the exhaust of your jet????