The blacks spin at 7200rpm, not 10,000. It's the Velociraptors that spin at 10K rpm.
The Greens run at 5400rpm. So you buy the greens for bulk storage or backups where speed isn't critical or where you want to minimize heat buildup (in an unventilated external enclosure, for example). If you want performance, buy the blacks. I don't really see a clear reason to buy the blues...
Don't the Blues have less platters? The Blacks are fast but the 1TBs, for e.g. have three 333GB platters. Most 1TB drives will have two 500GB platters. If that matters to you, it might be a difference to you.
Like sminlal said, the Greens are 5400rpm drives and more suitable as storage drives.
Weren't there problems with using the blacks in raid due to them dropping out of the array when they powered down, causing the array to corrupt? I know they have the 'Raid Edition' drives (RE3), but they are about twice as expensive as the blacks.
Hahaha, It means, it was a mistake that i bought WD green 640GB/64Mb cache. Black was a good choice. But after doing some research and put this question at forum. i fell, the best choice is VELOCIRAPTOR.