The Black drives are considered the performance line and have a few features that the Blue ( storage grade line ) don't have but there is not much difference in real world speed. They are both 7200RPM drives. The Blacks have a better warranty though. 5 years vs 2 years on the Blues.
Personally I generally get the Blacks for my personal computer just for the extra warranty. I have not bought a hard drive for myself since the flooding in Thailand last year that drove the prices up so much though. Actually I have not bought a mechanical drive for myself in a few years. The drives I generally use when I build someone a computer are the excellent Samsung Spinpoint F3 drives. Seagate has now bought Samsung's hard drive division but they are still made to the same specs as Samsung made them. They have one of the lowest failure rates of any consumer level hard drives.
It does not matter one bit when talking about mechanical hard drives. Not even the 10,000 RPM Velociraptors can saturate the 3Gb/s bandwidth provided by SATA 2. The even faster SATA 3, 6Gb/s standard is for SSDs. Even my 2 year old Vertex 2 SSD only uses the SATA 2, 3Gb/s standard. It still boots in about 15 seconds.
So when you see a hard drive marketed as SATA 3 or SATA 6Gb/s it's only marketing hype. It means nothing as far as drive performance goes.
Wow, so the only reason sata 3 is there is for the ssds?
Well the interface speeds are constantly getting better and better, which is good in many ways, it paves the way for faster drives. But mechanical drives have just about reached their limits as far as how fast they can physically transfer the data, because, unlike the "interface" which is 100% transistor in operation with no moving parts, mechanical drives use spinning platters and a moving read/write head. But, mechanical drives do have a memory buffer, which allows for the system to preftech data and store it into the cache, or as a place for incoming data to buffer until the drive can catch up, so to speak, which results in very fast or high "burst" rates for a few milliseconds. So, even though the drive may be way slower than the interface, that high interface speed is still desirable for certain operations, if only under certain circumstances, for very brief periods. But still, SSD performance would be hindered for sure if it not for advances in the interface its self.