Two options - first for a more stone grey than a blue-grey
Adeptus battlegrey - codex grey (leaving recesses, etc) and then wash black, retouch. Gives a nice colour.
The other I have done is adeptus battlegrey - lightly but thoroughly drybrush boltgun metal - maybe mixed with a bit of black if you want that irondust look - then wash blue or black depending on preference - looks like greyish steel, or gunmetal, rather than painted. Otherwise, see the above pictures. I rather like the 2nd picture, one on the left