Blood blister on heel a few days ago. Ignored. Now there's a v.Painful blister with what looks white beneath the skin. Not burst. Infected? Treatment?

Drain it. If you have no medical conditions like diabetes, etc, it is generally safe to drain a blister after 2-3 days to allow the skin underneath to develop. Use a clean needle soaked for 15 minutes in isopropyl alcohol. Drain, then cover with a sterile gauze pad and some antibiotic ointment. If discomfort doesn't resolve in a few days, if it becomes red or starts draining pus, see a podiatrist promptly.