Blue light has a shorter wavelength than other wavelengths of visible light (especially the ubiquitous red HeNe laser). This means blue light can resolve smaller features on the optical disc, which allows for the use of smaller bits on the Blu-ray disc to store information. This means that a Blu-ray disc can store much more information than a same-sized CD that uses a longer wavelength laser to read information.
Work Step by Step
When one uses shorter wavelength waves to read an image, more detail can be resolved. If you are curious, the reason that short wavelengths produce better images is that optical instruments are "diffraction-limited". Try looking up the term "Rayleigh criterion" for a quantitative treatment of the problem. This is discussed on pages 399-400.