Made by Gearbox Software and originally released in 2001 as an add-on to Half-Life, Blue Shift is a return to the Black Mesa Research Facility in which you play as Barney Calhoun, the security guard sidekick who helped Gordon out of so many sticky situations.
As the 2nd Addon to Half-Life, Blue Shift really does not have to add any new fangled weapons or elements to bring you a good Half-Life Experience.
Barney Calhoun, The guard who helped Gordon Freeman many times is as close to being a security guard in Half-Life as you will get.
Though it plays as An Stand-Alone game, Blue-Shift clears the air about the confusion of what occured Between Opposing Force, and Decay.