Im guessing bluestone is the same as crusher dust, which is the stuff they put on roads, only really finely ground.
We have used it before and found it helps drain away water and urine.
If you have mats on top, and the site drains well, I think you can get away with 4". But damp it as it goes down and give it time to settle and then level it before adding the mats and having horses in the stalls. If you are able to compact it at the time it goes down that would be even better. I would let it settle at LEAST a week, dampening it each day, before laying the mats.
If you can talk to a suplier you may be able to find a cheaper alternative, and they can tell you the best way to lay it so that it provides a good foor base and also will drain away moisture ect. You should ask about the screenings that Vida mentioned because I have hear they provide great drainage and if they are cheaper, go for it.