This can only be done in the upcoming games, X and Y.
In Pokemon X and Y, the Pokemon that can Mega Evolve should hold its Mega Stone (in Blaziken's case, the Blazikenite, which can be obtained from a special Torchic that can be received via the Internet for a while after the two games' release).
When the Pokemon is in battle, there will be a button on the 'moves' screen that will allows you to Mega Evolve the Pokemon. You can do this and attack in the same turn.
Of course, since the games have not been released yet, so this info may change before the games' release.
Remember, this can only be done in X and Y; in other games, you can't Mega Evolve.