The blocks currently being reccomended by and available from SJS/Thorn for use with Rigida CSS rims are made by Swissstop and is their blue compound Carbide specific pad.
I'ts certainly available in a slip in cartridge format for V brakes, don't know what others available. I've got them fitted to XTR V brakes with Rigida Grizzly rims on my new Sherpa which I collected yesterday.
Will let you know when I have had chance to try them in anger (when I'm back at work next week, my commute has a couple of sections with 20% gradients on it) they must work better than the old xt cantilever setup I've been using with various pads over the last 20 years on my Mercian!