Black tar-like substance on limestone

Trying to see if anyone could chime in on this limestone.

has about 5 blocks with similar ‘tar’. Tried everything in my arsenal. Straight SH. One Restore. EBC.

Don’t know what it is, but is it on consistent spots (same height) where it’s seen? Is it on a wall or fence?
I have seen signs stuck on some with a wide variety of fastening system if they didn’t want to drill in them.