GRU-ASU now with blocked seats for Black Sig. Other routes improved?
 

GRU-ASU was previously sold as PZ. Now that they are sold as LA flights there is a nice change - seats 1D and 1F are blocked by Special Services (Black Signature).
This route is every now and then full in PE, so this is a welcomed positive change from LATAM.

Any other routes with more blocked seats for Black Sig?

This is because PZ has fully converted to Sabre since February 2nd. So I guess they now able to use the same functionalities that regular LA flights present, such as this block for elite costumers.

Do you think they couldn't block the middle seat because of Amadeus?

It is not a question of the middle seats. The middle seats were always blocked in PE with PZ in rows 1-3.
The new now is that they block seats (1D and 1F) for Special Services on PZ flights on A320, just like seats 2D, 2H, 1J and 1L on 787 are normally blocked.
As pointed out by AlexMC, this is probably to be in line with general LA policy - but I quite sure PZ could have offered this before with Amadeus.

But all progress with LA/JJ/PZ integration is welcomed.

Agreed!