I'd recon your previous journey was short, allowing condensation to form and collect in the pipework. The collected water will have picked up some of the soot, then when you next started it up it was blown out the pipe and deposited there.
Either that, or you gave it a bootfull to pull away and some of the derv reek stuck to the floor.
Either way, nothing to worry about. Its a derv, they're dirty things.