1) full maturity in 50 Myr?

doubt it. While that would get out alot of the really blue O and B stars, I think the A stars can last longer. If I remember right, you prolly want upwards of a billion to get to maturity. Even then, there will still be alot of white and yellow.

2) I think you are observing the summed color spectra of all the galaxy. The blue drains out first. As to how much is correct, 100 years of observation of billion year timescale effects means there is still alot of if and maybe.

3) I know blue galaxies are common in the hubble deep field, so there are some waaaay out there. I also know that there are effects that can make galaxies very blue. Collision seems to be one of the big ones. I think that the current thought is that galaxies start mostly spiral, then collide with their neighbors, which tends to make them very blue for a while, and also tends to change the spiral to an elliptical, then they end up being an evolved mature elliptical.