Is this mold? What is the black substance between the wall and the ceiling?

Hello dear community,

this is not about any defects/warranty cases, but simply for me because I cannot classify it.

On the exterior wall between the wall and ceiling of our dressing room, black spots appeared a few weeks ago, which upon closer inspection and scraping off the paint turned out to be "black infestation."

The interesting thing is that the black infestation is exactly where, shortly before moving into the house, the joint between the wall plaster and ceiling plaster was "filled" and smoothed with painter’s acrylic to achieve a seamless look between ceiling and wall. I think this procedure is generally known. This was done at all joints between ceilings and walls (by the painter).

Has anyone ever seen anything like this?

Regards

I’ll stick my neck out and say that this is the [Mauersperrbahn] that was not cut...

I also suspect the blocking track. What kind of consistency does the "black infestation" have?

In any case, acrylic is not it in my opinion. To me, it also looks more like some kind of barrier layer? But what is a barrier layer doing on the ceiling of a living space? Somehow it doesn't make any sense.

But it makes sense, belongs between the wall and ceiling..

Ok, I don’t have a new building and I haven’t seen that like that yet. I only know it once towards the basement.