well blitz exposes you to a lot of games and so you can see a lot of different patterns and set-ups. The problem is being able to use what you learned from a blitz game to be a better player and with a blitz game you don't have the time to analyze all the variations. I play bullet, and I would say overall bullet has made me a better chess player because it has exposed me to so many new ideas and patterns, but it is not a very scientific way to being a better chess player
Does blitz really make your chess a bit weaker or sharper?

I used to think that it made you weaker. I no longer hold that opinion. I play alot of ten minute games here on Chess.com,and I find them useful, especially if you download them when you are finished, and analyze them in your engine. I do this win or lose, and it has really helped both my opening understanding, and my tactics.
i sometimes do that as well, it is very useful

It makes me weaker ......
When i stop playing blitz and return...... i do not take time to look deeper into the move order and not consider a 3rd option to play in a tough situation. When normally i look for 4-6 options.
"I think better when I am calm."-Mikhail Botvinnik
I also think better when I am calm, I do not like depending on my intuition that much.
I prefer blitz on only endgames or semi endgames. Does blitz chess really make you shaper or just a bit weaker?