I think rather than illustrating to Christos the function of the BL tier, one might rather illustrate the purpose of tiers in general. When I look at a list containing Pokémon of all the various tiers, why am I more inclined to use an OU Pokémon than a UU Pokémon? Because OU Pokémon are good in OU, as determined by the usage statistics. If we made BL the name for what is currently UU, and removed what is currently BL, the OU list would cease to simply indicate Pokémon that are good in OU, but rather Pokémon that would otherwise be too good in UU as well. This has no competitive relevance for a person wishing to play the OU tier, as well as being misleading.