This article explains the “black stuff”, also known by its trade name, Flintkote, underneath sidings found on houses built around 1950-1970s
I have done a lot of housework myself so far so when I rebuilt my entire kitchen (exterior) wall, I was perplexed to find tar-paperish black backboard stuff instead of the usual plywood exterior sheathing.
It was about 1/2″ thick and it resembled more a sheetrock than a plywood. In addition, these Flintkote boards were were attached to the house with nothing more than 1.5″ roofing nails and they could actually be cut with a knife!
What I ultimately found out is that Flintkote is an old tar impregnated homosote (compressed fiberboard) with the outside skin being tar paper.
Flintkote provides superior insulation than that of plywood sheathing, but plywood provides superior structural support.
Flintkote is no longer available so when you get a chance, I would recommend replacing it with 1/2″ or 3/4″ plywood.