I actually posed nearly the same question not too long ago. Here's that thread with some good responses.
I think blitz is good to stay sharp between tournaments, to work on tactics, to practice for time scrambles in longer games, and to perfect your openings. But for a beginner, I'd STRONGLY recommend longer games. In my opinion, chess improvement largely comes from deep, critical thinking and analyzing. Blitz (I'm talking 5/0 or quicker here) is mainly intuition played without much deep thought. So the majority of the time you're simply playing based on what you already know, not developing new strategies, ideas, etc.
I think blitz can be very helpful and certainly has it's place, but if at all possible I think playing longer games would be more helpful for you. If you can't do that or don't have the means, analyzing your blitz games after will really help. Play through the game and find where you or your opponent first left theory, and then next game try to improve on it. Blunder check with a computer for quick tactics and move onto the next.