Blockland.us has been registered since February 15, 2005. Badspot.us has been registered since June 20, 2006. The roblox website had been registered since January 29, 2004.
Of course we don't know how long these games were in development before the domain was purchased. Archive.org shows evidence that Roblox had been in some kind of private beta around January/February of 2005 (up until then the site was blank it seems). The public release of Blockland was also the second version of the game.
Short of both developers coming out and saying when they started developing, we won't know the exact answer. The best possible answer right now is "they both were publicly released within a few months of each other."