Once recorded onto video, especially digital video, the recorded data used to reconstruct the image uses the "Y" sample, or luma sample, for green values. The 'u' and 'v' values are technically a misnomer when applied to NTSC digital video, and Cr and Cb are more correct... the other two channels are red minus luma (R-Y) and blue minus luma (B-Y) aka chroma red and chroma blue.There's no more colour information in a green screen than a blue screen. Digital video works in YUV colour space, blue - yellow being the U axis, red - cyan being the V axis.
In most digital video formats, the luma is sampled four times in a four pixel wide strip. In pro formats, the chroma channels are sampled twice. In MiniDV and HDV, they are sampled once... which is why the edges on a MiniDV and HDV process screen shot tend to require.... tweaking. Note that the actual pattern for those chroma samples varies between NTSC MiniDV, PAL miniDV, and HDV.
Blue was chosen before the existence of video keyers, as silver halide in film is naturally sensitive to blue and the 'blue layers' of color film are the top layer of the film. Even black and white film is more sensitive to blue, so when shooting a matte pass using a separate strip of film, they got better resolution and separation by using a blue screen.Blue was originally chosen in the days of primitive keyers