The game was originally intended to be an extra feature exclusive to the Sega Dreamcast version of Half Life. But when the Dreamcast version was cancelled due to the poor sales and impending death of the Dreamcast console, the game's makers, Valve, Gearbox Software and Sierra decided to release the Half Life: Blue Shift as an add-on for PC version of Half Life instead.
If you go into the security office near the pistol range and activate the monitors, you can see Dr. Collette Green (from Half-Life: Decay) pushing a cart with the fateful sample ( a Xen crystal) on it down a corridor to the test chamber delivery system. In Half-Life: Decay, when playing as Dr. Green, you can see the security camera which films this image.
In the chapter Insecurity you can find a box in Calhoun's locker. If you return after acquiring the Pistol and shoot this box a chumtoad will come out. The chumtoad being a cut enemy from half-life 1 which was put into Half-life oppossing force and Blue Shift as a Easter egg.
The charecter of Walter Bennett seen several times in the campaign actually originated from a popular fan blog known as Walter's world and was put in as a tribute.
The name of the game blue shift is a reference to the scientific term blue shift which refers to the displacement of the spectrum to shorter wavelengths in light coming from celestial objects moving to the observer.