If you have a 12-cavity metal tart pan, you can use that instead of the muffin pan.
If your muffin pan is on the thin side, place a metal cookie sheet under the pan on the grill. Do the same if the crusts appear to be browning too quickly.
Raspberries or blackberries would be ideal as substitutes for the blueberries. Avoid using strawberries, however, as the moisture content will result in a too wet batter.