Playing blitz hardly ruins your normal chess knowledge and skill, but if it's your only training, you are without a doubt damping your own growth.
There are quite a few Gm's that play blitz pretty much on all occasions they got, and they are hardly weak.
If you find that you dont play like a grandmaster, even after 50 blitz games each day for a week, lol :) I'm sure you didn't play like a grandmaster before those blitzgames either...
Don't blame the blitz, blame the lack of real training and hard work!