The blast powerup is one of the first powerups you'll unlock. Keep in mind, blast notes are random and will be in different locations each time you play a song. Good songs for doing this on are Diamond Eyes, Spoonman, Cult of Personality and Give It Away.
If you've imported songs from the original Rock Band, you can easily get this on "Say it Ain't So" by Weezer. Skip to 3:19 in the video to see where I unlocked it. Again though, the notes are random, so it may not be there for you like it was for me.
Video credit: lildrchris25