Also, the broadcast forum or forums where engineers hang out would seem the more logical place to ask this, but I digress.
If you want to hear my guess as to why blue:
It is one of the three primary video colors, so it is easy for a video generator to, um, generate. And if your choices are red, green, or blue, plus black (often confused with "no signal") or full white (very bright and disturbing) blue is the least visually objectionable of those colors to choose.
Static is what you used to get in the abscence of a signal, and people found that very jarring, especially when coupled with noise on the audio circuit. Those were functions of analog signal flow, and TV's, VCR's DVD players, etc today are nearly all digital inside, so no static noise to generate.
If the TV sent just black, you may think the TV is off, so you want to send a color that shows the screen is getting *something* and that it is in color, versus B&W. That information alone can tell an experienced technician some important things about the set's status
Mark hit all the right points. The early video game systems hooked up via an A-B switch attached to the antenna receptacle on an old analog tv. It was common to see snow on one channel and black on the other. Same went for the early VCR's.
Blue is much easier to work with.