From left to right:
- CA brick (dark black; an early prototype?)
- LEGO-Logo Mx Black
- M-Logo Mx Black
- 2014 Modulex test brick
It doesn’t show well in this picture, but the 2014 test brick is closer to actual black than the normal grayish Mx Black. It also uses the newer Plancopy mold, which came into use after Mx Black bricks stopped being produced.
[Original post and comments on Flickr.]