Isopropyl alcohol works well for cleaning as well, and is probably a little safer than acetone, both for brain cells and fire safety reasons. I like the Starrett cleaner, which I believe contains some diacetone alcohol as well as a detergent-type component, and some other "stuff" as well. It has been my experience that the Starrett cleaner leaves a "silkier" feeling on the plate surface, IF you apply and remove exactly as the label says.
Note that granite is somewhat "porous" as well, and the comment about bluing residue leaching out of the surface is simply an indication that some of it remains in the surface cavities and micro-fissures of the granite, much like oil spots remain. I find that having a puddle of alcohol on the plate on a spot that is oil-stained is much more effective at removing the oil than just a wipe with alcohol. The alcohol "sucks up" the oil out of the surface, and similar action will probably help to get rid of bluing residue also. If it matters to you.