Greetings from Austria!

When I switch on the heating/air condition, black crumbs are blown out from the vents. The interior looks like a mess and they also make black stains on the leather. Who knows where they come from and how they can be removed?

Car is a 1996 XJS 4.0L convertible.

Best regards
Andreas

 

It is the dry rotted and falling apart foam seals in the ducts. Clean when it happens and it will eventually stop when theres nothing left

 

You could also try removing the wood and then removing the vent and vacuuming up any loose foam. Might help.
Mine went years ago from the dash vents, but when I opened up the footwell vents, lots of black crumbs fell out.

 

Pop the vents off and vacuum them out. That should end it.