Walls/Items Getting Black Film (not Mold/Mildew)

[Michele Fischer]

Greetings landlords. Have any of you come across a black film accumulating inside an apartment home?

Luckily this is not a unit we own, but we are trying to provide assistance in a situation where a renter is complaining that her stuff is turning black. You can see some photos here:(https://imgur.com/a/maZ1f8h). 

The wall photo with the string of lights had a vinyl cling on the wall in the shape of the black mark. If you rub the black mark it does not smear or go away or be affected in any way. 

Resident is blaming it on meth from previous resident but the unit has been tested for meth and it was clean. The unit has also been tested for environmental issues and moisture was below "dry levels" and indoor air quality levels were okay though CO2 levels were just above guidelines in some rooms.

The tenant is known to use a lot of candles, we're wondering if this could cause this.

Does anyone have any ideas? Or where we could look to find out?

Thanks!