The "Blue Star" achievements for each song are unlocked by playing the song 5 times. A quick and easy way to do this is as follows:
1) Go to Options (accessed by hitting RB a couple of times on the Song Select screen) and set the difficulty to HARD.
2) When the song starts, smash a bunch of random buttons to fail the song immediately.
3) When the girls fall to the stage, hit Start (this will shave off a couple of seconds).
4) On the menu that appears select the option on the right which is 'No'.
5) The next menu is about saving the game. Unless you are quitting the game, done with the song, or changing characters: select the right option 'No' to save time.
6) Select the first option on the next menu to restart the song.
7) Return to step 2.