It is a counter play because Lulu is able to stop just about any champion ability before it is fully casted if caught in time.
If Blitz were to grab Lulu before she completed her polymorph, she would be pulled in and the spell wouldn't go through therefore cancelling out her ability also.
Due to this, it's not a counterpick because Blitz and Lulu are both capable of just about the same thing (i.e. speed-up, silence/polymorph, knock-ups.)
Just because Lulu can stop Blitz' hook if done in time doesn't mean it's an automatic counterpick.
Blitz can also stop Lulu too if caught in time.
Honestly, Blitz and Lulu are a fairly even matchup due to their skill sets.
With Sivir, it's not really a counter pick because Sivir's only way of avoiding Blitz is her E (spell shield.)
And with that being said, her spell shield takes very good timing because if done too early or too late, you've been grabbed.
You could ult and try to run away but even still, if Blitz grabs you, your ability will be stopped before it's fully done.
Even if you were to play a champ that's a true counter to Blitz, doesn't matter if you don't know what you're doing.