Why all blocks at Blockchain are relayed by Unknown now? only last week I could study which mining pool signed latest blocks.
Today this option turned status to Unknown

https://blockchain.info/

Any idea?

(formatted table)

Height  Age     Transactions    Total Sent  Relayed By  Size (kB)   Weight (kWU)
491728  31 minutes  352     196.23 BTC  Unknown     1,002.67    3,992.19
491727  31 minutes  266     920.01 BTC  Unknown     134.31  515.14
491726  33 minutes  1447    2,900.32 BTC    Unknown     1,017.02    3,996.92
491725  39 minutes  290     822.51 BTC  Unknown     132.29  494.91
There was an update to the site that changed its behavior to no longer properly identify who relayed blocks.

Until they fix the issue, you can use other blockexplorers to find the information.

Given your stated usecase, I suggest you look at http://coin.dance/blocks