Recently Bizarre Creations announced that there is no DLC for Blur in development, but they do have plans for DLC in the future. According to lead designer Ged Talbot, throughout the year or more of development not a single team has worked on any sort of DLC.
Bizarre Creations assures everyone that DLC is coming, just not on the release day. So everyone who buys the game will have to be happy with the 50 cars and 30 racing locales coming on the disc for a little while. (As well as pre-order bonuses depending on where you buy the game.