Stone Grinding/Sanding Discs (if there is such a thing)

Diamond disk. The cheapies will do for concrete blocks.

You can take the bulk off with a hammer and chisel. Or indeed do the whole job with a hammer & chisel & save money.

IIRC angle grinder discs for metal will cut stone but not vice versa. I don't have an angle grinder and have never felt the need for one.

I found to my surprise that some discs meant for removing rust from car bodies worked really well for this. Gave a nice finish.

Bill

BURN THE HERETIC!!!

Indeed I have done this, but it's pains-taking and tricky, because the texture of these blocks is very knobbly so getting a smooth result is next to impossible, even though I'm something of a sculptor - at the risk of being accused of immodesty - if I say so myself. I suppose I could use stone *cutting* discs to x-hatch the proud surfaces and then chip them back with a chisel if the worst comes to it, but I'll check out the other suggestions first.

Stone polishing discs are/were available. USed a lot by stone masons at one time.

Don't use a cutting disc. It may shatter if you attempt to use the surface to polish

Yup, got one today locally, although these discs are rarer on the ground than the other types you get. You can also order them via suppliers like Toolstation.