Many centuries ago (okay this century but a long time ago) I bought a number of scenic items from Skytrex.
Amongst those items was a packet of bound stacks of breeze blocks (cinder blocks are similar).
Base sprayed Citadel Chaos Black.
Dry brushed Vallejo Black Grey.
Dry brushed with a 50/50 mix of Vallejo Black Grey and Vallejo London Grey.
Dry Brushed with Vallejo London Grey.
Dry brushed with Citadel Terminatus Stone.
Given a wash with Citadel Nuln Oil (matt).
I should really have painted the straps, probably an off white.