Drain it, cover it up, and keep running.  Are you wearing good, non-cotton socks?  You shouldn't get blisters.

At distances beyond about 10K, if I don't use Body Glide or equivalent on my feet, I will occasionally get blisters even with good synthetic socks and well fitting shoes. Go figure. Neosporin and band-aids seem to work well at clearing them up. Usually I have to strap some athletic tape over the Band-Aids, otherwise they will slide off as soon as my feet start to sweat a bit. I will normally run straight through blisters, as long as they're protected and they aren't causing enough pain to affect running form.

Blisters are minor injuries, but don't let them get infected. Earlier this year I got several infected foot blisters (from backpacking) and those were sufficiently painful that I could not run for 4 days after I returned. I tried, but my running form was definitely affected, and I did not want to risk a secondary injury. In that case I had to crosstrain those days and wait for them to heal.