While renovating our kitchen the electricians opened part of the exterior wall. It is a plaster wall with metal wire lath in a 1920s-built home, but I can't identify this darker material behind the plaster.
The electricians stopped manipulating the material and asked if I wanted to have it tested for asbestos. The testing is in progress but I'm not sure what this material is to begin with.
It does look fibrous but it is dark. It almost looks like fiberglass, mineral wool, or even cardboard, but it seems to be comprised of strips that are nailed to the studs and/or furring strips.
(The gap in the material was not cut as part of this renovation. A previous owner had bathrooms installed or updated above and below the kitchen; we think the plumbing visible in the pictures is the vent stack and/or sewer line, and the plumbers must have cut this gap in order to install it.)