Just depends on the monitor/tv manufacture. Some default to blue...some to green and some to black when signal is not present.
Funny monitor story...
In the mid to late 1990's CNN Atlanta was slowly replacing the original CRT monitors in the newsrooms, edit bays and control rooms. As engineers and contractors replaced the sets, many of the old ones ended up stacked in corners, hallways and break rooms. As you can guess...some of those monitors "walked away" and found new homes as video game monitors, extra tv's for kids rooms or grandmas house...
It was not unusal to find some low paid CNN VJ or production type watching an old Sony monitor...with "CNN" burned into the CRT in the lower right corner of the screen.
(For you youngsters...CNN was not as graphic intense back then....but the yellow/orange CNN bug was always there, so the image would "burn in" the screen after being always on screen year after year.)