Had the same problem. People walking straight off factory floor, grease on their boots, blue tiles become black. Used a de-greaser from local supplier, brushed in, extract with prochem fibre & fabric. Other areas treated with power-burst. You will not get them back to new and its important to pre-qualify this with the client - 'managing expectation' its called. Also you can't use the really aggressive stuff because it might break down the bituminous backing (same stuff as the soiling = hydrocarbon products). Get it down quick and extract, repeat if necessary, dry it quickly if possible to reduce any wicking from the backing. Good luck!
