gakukid991 :
i don't have an usb 3 port so i can't get an external hard drive bz i will not be able to use it at full speed
Doesn't matter much. Hard drive read/write speeds are around 140MB/s, USB3 is around 500, and USB2 is around 50. So while USB2 is slower, for most tasks it's not noticeable. Watching a movie is around 3MB/s if I remember right, so it won't be a problem at all.
If you get the 2.5" drives, you can usually fit two inside the case (as long as you have enough SATA ports on your motherboard). This lets you put Windows on one drive, and movies/music/documents on another, so that if something bad happens, you only lose windows and keep your other stuff safe.
But if you're going for 3.5" then blue is probably just fine. Black really only makes a lot of sense if you go for the 2.5" drives.