Tiers are basses on usage, and not power. This means to say that basicly Mega Medichan isn't so high as it is not use so much. BL is however a higher tier than OU, so this does still make seance.
^ The lists are a bit outdated, but the information on tiers is still legit.
^ This list may not agree with what Smogans was at the time, but Smogan is not affiliated with The Pokemon Company, so all lists, even theirs, are just personal opinion.
^ This source is completely reliable for the question at hand.