I still believe if BL were to be rated it'd be E-10
Online interactions are never rated, unless it has a system which blocks almost all vulgar and inappropiate content or the online services for the game is just a set of Leaderboards and/or downloadable content.
In other words, it's impossible to rate Blockland unless it did get a story-mode where you try to find a golden brick. In that case, it would probably be E-10.
And be even more of a rip-off of Lego in some regards.