Just a dissenting opinion here..
All 3 of my kids have crocs and live in them all spring and summer. They have never had ONE Croc-caused blister. We also did 9 days in WDW in 2007, all 5 of us in Crocs all day every day, not a blister between us. I think it just depends on your feet.
Now, if you buy knock-off crocs, ANY other branc of croc-type shoes, I have found, you WILL get blisters. The problem usually lies in the rivet that holds the heel strap on. Real Crocs have a way of sinking the rivet into the shoe so it doesn't rub your feet. Other Crocs don't. My DD5 got horrible shred-the-feet blisters from a cute pair of fake Crocs given to her by a well-meaning but penny-pinching relative. Never again!
Crocs all the way for us!