In my opinion, I dont think the "Blacklist" is a completed list in the story. Sure he has most of the names down.
Even members of the "Cabal"maybe.
But as i see it, i believe the list is formed and ranked as Red discovers these people. Hence the list is probably one that is already compiled in the "future" after all blacklisters have been found. Another example of this is in
episode "Berlin", where Red, up to that point, only knew someone was targeting his interest, but not who. Hence, the Blacklist name and rank must only been added after he known.
Another alternative idea: Red probably already knows about the Decembrist, as every other blacklister, and their work but not their identity. He has ranked them based on what they do. Hence he is hunting/catching/killing those he needs or poses an imminent threat to Keen or his Empire.
My wild guess, is he himself may be on the list, ranked #1.