So, like everything else we know about WP8.1, it is just a rumor. Though MS has provided a select few developers data that states there is a FB Blue, but nothing has been announced about it yet. As far as I am aware though, this is still a MS and FB joint effort (the current FB app, though made by MS is supported by FB.. so to an extent it is "official".. )
That being said it will have to release with wp8.1 to allow the hooks into the OS for FB integration with the OS that the current API that FB app is written on doesn't support. There are no more details than that really until hopefully BUILD where this is all announced.