ᐅ Wallpaper peeling off near the fireplace – Is it soot or mold?

Hello everyone,
I noticed a strange smell in the hallway, somewhat like burnt electronics.
Following the smell, I identified the wallpaper on the fireplace as the source.
The last time the fireplace was used was about a month ago. The fireplace is only used with a wood stove.
However, the smell has only been noticeable for about two weeks.
The wallpaper had already started to peel off in several places and formed "blisters."
I then cut open the wallpaper to inspect the damage.
At the top, there appears to be a connection; it looks like there is a cover that is not completely sealed.
I believe the black spots are just soot. It also smells strongly of smoke.
My partner says it is mold.
I think the cover is just no longer properly sealed and needs to be replaced.
What is your opinion?
Regards, Till

I’m quite sure this is not mold. The chimney is leaking through that faulty spot and is seeping behind the wallpaper, soaking everything. Of course, this needs to be fixed.

Mold looks different; I would rule that out!

Thanks for your assessments.
Now the goal is to close the hole.
The hole is on the first floor, and on the ground floor there is a wood-burning stove connected to the chimney.
After consulting the chimney sweep, I was advised to brick up the hole instead of buying a new cap. Since no stove will be connected there in the foreseeable future, I would like to proceed with that.
But what is the best way to do this?
The chimney sweep’s answer was simply "just brick it up."
Which materials can I use to seal the hole airtight?
Regards, Till