Blame John was a trend in ROBLOX that came to be after the release of the hat and gear, Blame John Tie and Blame John Sign, in 2010. The 'John' in the saying refers to John Shedletsky, the most popular administrator on ROBLOX. The inceptor, xLEGOx, reported the trend had originated from an out of context quote by him in a thread.
Blame John could mean that Telamon, now called Shedletsky since 2012, has enough control over the website to do potentially questionable actions. If a new feature was introduced that gained controversy, the saying would be implemented in some time.