Experts Retrace a String of Mishaps Before the Blackout. At 3:32 p.m. on August 14, a transmission line sagged into a tree just outside Cleveland, setting off the events that led to North America's biggest power failure. By James Glanz and Andrew C. Revkin. [New York Times: NYT HomePage]
This is the best explanation of the blackout that I have read. It helps if you have an electrical engineering degree. It also explains why southern Ohio(where I live) was not affected. The short version of the theory is that First Energy(Cleveland) was having transmission line problems. This problem aggravated an overheating with the transmission lines to AEP(south of Cleveland). When First Energy lost a second transmission line, AEP circuit breakers tripped on the lines to First Energy. This walled off First Energy from the south and the system quickly started to pull large amounts of power from the north. In a cascading patttern circuit breakers protected the equipment and shut down the transmission lines from Michigan through Canada to New York. It was interesting to note, that New York was exporting a large amount of power through Canada at the time and went down because they could not slow their power generation fast enough.