The Builders League United, or BLU, is one of the rivaling groups in Team Fortress 2. The BLU team is currently helping the User Battle Force due to the Reliable Excavation Demolition (RED) team being on the villains' side.
The members of BLU generally have higher base Attack stats than the members of RED. To compensate, they have lower defenses and higher Special Attack, which does not help the TF2 metagame in any way. Although the base stat totals even out, RED has a far superior stat distribution. The members of BLU also have more weapons, but RED tends to have a better weapon distribution as well despite being trumped in options.