Pushing off or grinding in. Doesn't have anything to do with pronation problems.
Blisters on the fore feet (balls of feet, toes, etc.) indicate pushing off; blisters on the heels indicate heel striking, scuffing.
Concentrate on bending your knees more (but don't go overboard, since you don't want to over tax your calf muscles) to help you avoid heel striking, but also help you to not push off and get over your center of mass/gravity.
Try again, and let us know how it goes this time. With more and more practice, blisters will be a thing of the past. We all tend to get over them eventually.