1 more time with this
no "Unknown" is not 1 "pool"
it is all of the miners that solo, some private pools and propably some botnets.
even some of the recognised pools are in that group
everything that blockchain.info doesnt "know for sure" and they use some "best guess" way that often misreports deepbit blocks
there is a big botnet in ther often referred to as "mystery miner" who has a habit of making 1 txn blocks, and while this is a small concern it is not a big issue (although some would have you believe otherwise)
truly this subject has been covered many times
have a look thru the forum http://blockorigin.pfoe.be/top.php
is usually more accurate than blockchain.info kinlo has done great job of better identifying which pool actually found the bock - unfortunately its a little behind atm.
hope that is a help