At the present time, the only way we know of that light can get shifted toward the blue end of the spectrum is the Doppler effect ... wavelengths appear shorter than they should be when the source is moving toward us.
IF that's true in the case of the Andromeda galaxy, it means the galaxy is moving toward us.
We use the same reasoning to conclude that all the galaxies whose light is red-shifted are moving away from us. That includes the vast majority of all galaxies that we can see, and it strongly supports the theory of the big bang and the expanding universe.
If somebody ever comes along and discovers a DIFFERENT way that light can get shifted to new, longer or shorter wavelengths, then pretty much all of modern Cosmology will be out the window. There's a lot riding on the Doppler effect !
