From TF2 Wiki
The Builders League United is a holding corporation founded by Blutarch Mann specializing supposedly in construction operations. In reality, from behind a myriad of sub-fronts, it secretly controls half the world's governments, led by an evil female overlord known only as 'The Administrator'. Beneath the convincing exterior of their bases of operations can be found computer racks, satellites, rockets and missile installations.
The other half of the world is just as secretly controlled by their rivals, Reliable Excavation (and) Demolition. Each corporation administers its half of the world with a multi-disciplined army of paper pushers. For any problem lacking an obvious bureaucratic solution, mercenaries are contracted to address the situation through a massive application of force. Conflicts between the two companies began as land disputes between their original founders, though the current reason for their ongoing combat, with both companies now under the thumb of the Administrator, is unknown.
BLU and RED share an intense rivalry, with nearly all encounters between the two quickly escalating into brutal and often fatal violence. Inter-company friendship is also strictly forbidden by mercenary contract. Despite this, the BLU Soldier and the RED Demoman became fast friends after meeting at a projectile weapons expo. However, once this was brought to the attention of the Administrator, she pitted them in deadly competition with a mysterious new weapon promised to the winner, twisting their former friendship into a bloody war.
BLU favours a more industrial style of architecture than RED, and while its dress code is rather liberal in terms of clothing, it is required that employees adopt a blue colour scheme for easy identification.
- Although the acronym BLU means Builders League United, they are usually the attacking team in Payload maps such as Goldrush, and Control Point maps such as Gravel Pit. Whilst the acronym for RED fits in more well with that role, being Reliable Excavation Demolition, they are usually the Defending team.
- Valve has often depicted players using Macs as the BLU team, as in the Mac update trailer. This is probably due to the lighter colour scheme of the team and their more streamlined style of buildings.