Blitz Cred is added at the end of a song based on the number of stars you get and what tier the songs is at. To figure out the creds multiple the following base amount (10, 15, or 20) by the number of stars.
Song Tiers 0-1 have a base amount of 10 creds.
Song Tiers 2-4 have a base amount of 15 creds.
Song Tiers 5-6 have a base amount of 20 creds.
For example, you will get 80 creds by getting 4 stars on 5-tier song. Gold starring is considered 6 stars, so 120 is the max cred per song.
Assuming you always play 5-6 tier songs, then this achievement should come after getting 500 stars or 125 4-star songs. That's a lot of songs, but you don't have to do anything out of the ordinary to grind for it, so it's just playing the game for a while to eventually get the achievement.
You can also get Blitz Cred from Score Wars. You get 50 Blitz Cred if you win and 10 if you lose.
Notes: I'm only half of the way there, but this is my current understanding of this achievement and I'll update it as we learn more. (See http://www.rockband.com/forums/showthread.php?t=231419
). If you play offline or get disconnected to the Rock Band Central server mid-song, the creds will not count ( http://www.rockband.com/forums/showthread.php?t=231429