What are these black spots?

[Christian Nachtrieb]

Can anyone help me to identify these black spots? These are located on the backside of the (very) old baseboards on the plaster walls of an older condo. Wife is convinced this is mold, I'm not so sure. This location here is in one of the bedrooms, nowhere near the kitchen or bathroom, so I don't see any immediate ways in which water could've settled up here. You can find these black spots in a couple other rooms like living room and another bedroom. It looks charred to me?

Any idea what this could be?? We're having a mold person come and look at it, but I'm weary that he's just going to say YUP that's mold, hire me to remediate it.

Thanks BP!

[Jim K.]

@Christian Nachtrieb

Christian, glad I saw this. That's rust and oak tannin stains. The baseboards are oak. Nowadays, the most common way to attach floor trim is 2-inch 18-gauge brads fired in with a pneumatic nailer. Back in the days, you used 8-penny 2 1/2 finish nails. The nails rust quite a bit and react with the oak over time to leave those black stains in the plaster at the attachment points.