I hope you have visited as this occur to you child.
I would go either to urgent care, or right to a podiatrist if you can get in right away. I have a similar problem right now, but the blister part is closer to the side of my heal. I finally went to urgent care and was told it is cellulitis which can be very serious if not treated. Watch the area to make sure there are no red lines going up your foot or leg which could mean infection. The doctor will also rule out blood clots. Go in right away!
You require urgent medical attention.