Blue is a compromise between Blacks and Greens. WD Blacks are 7200 RPM, delivering more performance at reduced battery life, while WD Blues spin at 5400 RPM, the performance is lower but the battery life you get will be longer. That being said is most scenarios you would't notice the difference between 5400 RPM and 7200RPM. In Terms of gaming you will only load the game a little faster.
WD Greens are more suited for storing large amounts of data, that do not need to be accessed quickly (movies, music etc) at a reduced power level.
FYI most laptop hard drives are 5400 RPM, so if they are shipped with WD drives they will almost always be the blue version.