You say that you've started running barefoot, how recently? If it's very recent, your feet are not accustomed to that sort of serious strain. No callouses have been built up, they're as soft as a baby's because of the foot coffins we wear all day long.
Start out slow, work up to the distance you can normally run while wearing shoes. Consider this a 'new workout' since you need to build up tolerance, primarily callouses and running style, where you have probably not had them in the past.
You said in one of your later comments that you were able to do 33km in 3 stints, so I'm going to have to assume you could do 11km at any given time. That's just shy of half of a half marathon (quarter marathon?). Essentially, you doubled your distance to reach that half marathon, where you experienced blistering. That doesn't seem all that strange to me. I know when I switched to running barefoot, the distance I was comfortable running was cut way down until I got accustomed. Granted, I'm no marathon runner and I don't pretend to be, but I went from 10-15 km to dying after 5 km.