Black editions have an unlocked multiplier. That means higher overclocks. I understand it also uses higher grade chips. Chips are speed-binned - when they are tested, the processors are sorted (binned) by how fast they will go at stock voltages, etc... - the higher quality the components, the faster they'll go.

Basically, the Black editions use better quality chips, which require less voltage to operate at the rated performance - e.g. 5200+, and so will overclock better with higher voltages. Combine that with an unlocked multiplier, and you have the core of a nice overclocker's system. I'd pay the extra cash for one, because I'm into overclocking. But that's just me.