Pictures?

Black coated

Sorry, the website is a disaster on mobile, took 5+ times to get them to upload 🎉

Its probably roofing asphalt intended as a moisture barrier. We have run into this once or twice. As a moisture barrier, it is not very effective. The mason should have installed felt paper over the studs, but it may be that the masonry wall went up first, so there was no choice since the studs did not yet exist..

Thanks! There actually was a black felt paper layer, but I tore it off. You can just sorta see it in the edges of the pic above.

I was thinking it was something tar like, but hadn't heard of asphalt on masonry before. This seems really likely!

For that era it might be coal tar pitch which contains carcinogens so you should encapsulate it with a coating that won't soften it.

To determine if its coal tar pitch or asphalt put a bit of it in lighter fluid. Coal tar pitch will turn the fluid a bright yellow-green and asphalt will turn it brown.