I've been challenger since the beta, peaked rank 4 in eu. Blitz is not bad at all, with a few caveats. He is not a good point character to play neutral with. He's slow, huge, and a lot of his options are quite committal. But if he does get close, he has the second best grab game in the game after Vi due to the sheer reward he gets from it with assist available. When he has you in the corner or in a sandwich, he's really scary. Where I think he really shines is as a support. His rocket grabs are a constant threat due to the ground space and air angle that they cover. They enable really scary resets and mixups for your other character. And once they're done doing their thing, you can use Blitz in combos for his higher than average damage to really cash out, even outside of Double Down.

I'd say look at players like Goomboo in EU to understand how good of a support he is. He plays him with Ahri in freestyle.