Blue lights are often used by theatre techies back stage and in other technical spaces like sound and light booths.
Blue light is much easier to mask from the audience than white light and it is easier to work with than red light.
If the blue is as far down the visible spectrum as far as Royal Blue is (~455nm wavelength) it will be getting close enough to NUV that some things will floresce under it.
The utility of monochromatic lights is highly subjective with some swearing by them and others who swear at them. Some users just don't like them and never get used to them. I also believe that practice may make monochromatic lights more useful for many users, but that's true of white light, too.
I used blue lights for years (and years) in technical theatre and my skills with them improved a great deal in that time.
On the other hand -- they are not for everyone.