BL stands for "Borderline", if a Pokémon is ranked in these tiers it means that they are not used enough to be placed in a higher tier, but brings imbalance to the tier below. For example a Pokémon ranked in the BL tier is considered not to be used enough in the OU tier but brings imbalance to the UU tier.
>A BL Pokemon isn't necessarily weaker than an OU Pokemon. BL Pokemon, however, are used less for reasons such as requiring additional team support or simply being outclassed by another Pokemon in a specific role. It usually takes only one such inadequacy to explain why a given Pokemon is BL and not OU.
Pokémon who are listed in the BL tier can only be used in Ubers or OU matches.
Pokémon who are listed in the BL 2 tier can only be used in Ubers, OU, and UU matches.
Pokémon who are listed in the BL 3 tier can only be used in Ubers, OU, UU, and RU matches.
List of tiers (not battle formats):
OU - (Overused)
BL - (Borderline)
UU - (Underused)
BL 2 - (Borderline 2)
RU - (Rarelyused)
BL 3 - (Borderline 3)
NU - (Neverused)
NFE - (Not Fully Evolved)
VGC - (Video Game Championships)
(Borderline tiers aren't tier in which you can battle, as stated above their only purpose it to separate Pokémon who are considered not to be used enough to be in one tier, but brings imbalance to the tier below for reasons which may include:
- The Pokémon being too powerful;
- The Typing of a Pokémon etc.)