It's the copper plating that was affected. And definitely a worn die strike. Many Copper plated zinc cents look bad. That's why here on CoinTalk we call them Crappy Zincolns
Here is some great information http://www.error-ref.com/blisteredplating/ - Mr.Ed (Specializing in collecting errors since 1985)
...the problem is two-fold: they will keep them in circulation after they corrode, and they exacerbate the situation by making more of them every year...Spark