It is important to know that some of the older blues used more platters to make up the same capacity point. The blacks(or any drive) with higher density platters had been faster.
The specs being the same does not mean much because cache is only useful for a short time, once it gets down to actual performance of the platter/head combination things can change.
It is all additional specs. higher density platters are faster and faster rotational rates reduce latency(and a bit of drive firmware programming helps).
I know my WDC 1TB blue(the older model) was not near the black for performance(but it was still faster than the older 640gigabyte WDC blacks) as a boot drive(my old 1tb Seagate also took it to town in real world performance, but also never lasted nearly as long had many of this model fail).
For the OS, get the fastest you can(SSD if money would allow), but for storage, drive speed does not seem to have much of an effect on performance.