As said earlier, it's not really a fork of the blockchain, it's a totally new blockchain with 1 block containing everything but the FBI's coins.
And as has been pointed out, to make a totally new block chain with 1 block containing everything but the FBI's coins is a hard fork of bitcoin creating an alt-coin that happens to be based on a snapshot of the bitcoin blockchain at one point in time. You can call it a new blockchain if you want, but then it is a new blockchain based on the bitcoin blockchain which is a hard fork.
By the way, I think this is a great idea. More power to people who want to experiment and see where the experiments go. That is what is great about open source, you don't have to sit around debating something, you just have to go and make the changes and put them out there and see who adopts it. It is the perfect way to test ideas and let people vote with their money to see what they prefer. And it will put an end to the endless debates (well, probably not) from people who want to remove fungibility from bitcoin, so PLEASE go for it! (And go for the forks for ID-coin, inflation-coin, not-fairly-distributed-coin, use-a-'useful'-POW-function-coin, etc are great ideas too so we can see how they all flourish...or not).
Anyway, I am really excited to see it! I'd still like to know when this will launch. By the way, I'll be happy to sell my coins in this new alt-coin to those who believe that bitcoin should not be fungible once people are buying non-fungible-fork-coin.