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If the powerwasher thing fails, and I think it will. I have tried it b4 on a similar issue, you could try TSP which you can buy at home cheapo it's power and is water activated. It works pretty well on all kinds of concrete stains, also muriatic acid is effective in removing stains from concrete though I have never use it on a coloredblock/paver. If forced to try acid test it on a scrap that I'm sure you have laying around. Be sure it's completely dry before you make a final judgement as to whether or not it is safe.
That wall should be lined with firebrick. If the blocks are glued together, the glue will fail over time from the heat. I wouldn't worry too much about the soot- its a real working fire pit... what did you expect?
I would try oven cleaner, or whitewall tire cleaner, but test it out on a small area first.
Maybe you can find someone that uses a media blasting unit. The ones they use to remove graffiti. We use a system called MaxxStrip to remove calcium and minerals from everything from pool tile to stone. The media is just a little different.
It is a fire pit. It would just look like an unfinished planter if it weren't blackened. I wouldn't try to clean it unless you aren't finished with the job and someone on the crew decided to burn trash in it or something. It should be up to the owner to make the first black marks.
I like poolboys idea, Hey Poolboy would blasting affect the taumbled finish of the block? Maybe smoothing it out? I haven't seen what you refer to but it sounds intersting. Does in remove efflouresence staining?