I'm redoing a shower and one wall is brick ("slump block"). 40 years ago it was mudded and the tile affixed to that. It took me 3 hours with a chipping hammer to get it off. After exploring some alternatives such as furring strips and Hardibacker I seem to have come full circle to mud wall again. The stuff that came off my wall is white in color and hard as a rock, but I can't find anyone who can identify it. Meanwhile, I am looking in amazement at a page on this site entitled "Mortar bed for walls" showing mud being put over plastic, with no obvious reinforcing.
Sorry for being verbose; bottom line - what's the best mix to use for mudding a brick wall? Any suggestions appreciated.

Of course there's reinforcment (wire lath) otherwise it wouldn't work.

The mortar over your block could be plaster/parging and the tile set to that .... or just white mortar.

Check it out.
http://www.ontariotile.com/mudwall1.html

I don't have to .... I did the job.

Lath was used.

In a couple of pics that show the end of the tub you can see the lath on the walls.

FWIW, I've enjoyed this little back and forth, interesting. Good luck on your wall, Frank.