If the unit sat for a while and critters (bugs, spiders, etc) made a home, yes, some initial soot would be normal, but if if were to continue beyond a few minutes, you may need to re-adjust the burner. On our unit, it's as simple as a screw and slide type event on the fuel/air delivery pipe.
Computers manufactured by companies such as IBM, Compaq and millions of others are by far the most popular with about 70 million machines in use worldwide. Macintosh fans note that cockroaches are far more numerous than humans and that numbers alone do not denote a higher life form. -NY Times 11/91