Are you using sugar in your recipe? Sugar can cause excessive bottom browning and blackening because it caramelizes, especially at high oven tempereatures, and often before the rest of the pizza is done. Another possible solution is to use a screen, or a screen in combination with the stone. The metal of the screen acts as a barrier between the pizza and direct heat and protects the bottom of the crust from overcooking. If you want, you can slide the pizza off of the screen onto the stone once the crust has set. Using the screen also eliminates the need to use any dusting agent. Another possibility is to lower the oven temperature a bit and let the pizza cook longer. You can also try raising the position of the stone. If you are using too many toppings, the crust will often finish baking before the toppings have finished cooking, and will then start to turn black if left too long beyond that point.