Sand blasting is not a do it yourself project. You need to get someone in and it can be pretty messy. As an aside, sandblasting is usually used to take paint off of stone or brick. Not really sure if its going to do any good on bluestone - is the theory that there's some kind of build up on the stone that would be removed? Have you tried using a color enhancing stone sealer? You can buy a small container of it and try it on an inconspicuous spot. Also, there might be some kind of stripper that would dissolve the grime if that's the problem. Talk to an expert at a stone supply store, if there are any in your neighborhood.