Black stains on FG insulation

Hey all,
I keep seeing black stains on FG insulation and need to validate what I THINK I beleive it is… (sorry no photo handy)
Common scenarios are in garages or outbuildings with VB installed but no wall or ceiling finish.
I used to think this was soot that found its way through holes and overlaps in the VB and attached to the insulation now having condensation thereon, since it was fairly common in garages with wood stoves.
Often newer similar buildings that have (or never had) combustible units (woodstoves, oil/gas furnaces).
I’m stumped… help!