Remember learning "Roy G Biv" back in HS? It stands for the colors of the spectrum: red, orange, yellow, green, blue, indigo and violet. Red light has a shorter wavelength than blue. Infrared is literally "below red" and is too long to see. UV is shorter than violet and is too short to see. Blue lies in between and is one of the shorter wavelengths the human eye can perceive.
A shorter wavelength beam of coherent light (ie laser) can read a smaller pit than a beam with a longer wavelength, like the current red ones in todays gear. That's why Blu-ray is an advance over current DVD technology; in addition to more advanced data compression, it can simply fit more pits onto a given area.
The main disadvantages has been higher cost and a lower service life for blue lasers than for the red lasers in use now. The difficulty in creating an economical blue laser is the main reason Blu-ray has taken so long to get to this point.