From my experience - I have a 2tb Black since they came years and years ago but also have fiddled with a 2tb green at a friend's- it depends on what you want as the others have stated.
The blacks are faster and in some cases a lot faster and i used to use mine for the OS and was really good. Nowadays though with the SSDs around i don't think there is a reason to go for a black. Go for a green or red (3 years warranty instead of 2 that the green has) if you want it for storage. For OS buy a nice SSD they are cheap and you can get a 250gb for a good price. Plus the blacks are so expensive you can get an extra tb or 2 in a green or probably a red version.
Pros of black:
faster (with an SSD you won't need this attribute)
5 years warranty
better access time
Cons of black:
needs more power than the green
a lot more expensive
only up to 4TB no option for a 5TB or 6TB like the red/greens
noisier than the green and red (not as noisy as velociraptors though if you have one)
I will mention some notes for the green:
The heads are getting parked and going to a kind of "sleep" mode and once you access it it needs some milliseconds to power up before you are able to access anything, but if you are used to how the usb external ones work then this will be fine.
Pricing: in most cases and while in stock the green 6TB version is close to the 5TB version of the RED and close to the 4TB version of the Black so decide what your needs are. Also because the 6TB is the bigger WD has and is the biggest out there (for now see below) the are expensive, you could potentially get 2 x3 Greems fpr 79.99 each and you would save 30 pounds from the 6TB one. Of course only if you have the space in the case.
Also if you are not in a hurry, seagate just shipped the first 8TB drive so i would expect some competition between the 2 and probably some price drops on the current wd. I am waiting for that i could use a new HDD but i will just delete a few games that i havent touched since 2009. Have fun whichever you pick
Edit: Now i noticed you were asking about the blue, my mistake i am sleepy and didnt notice. The blue is faster (7200RPM) than the red/green. I dont have much experience with the blue so maybe someone else has and can give us their opinion