Torches with blue light

 

— one of the hues used in the main three-colour "RGB lights" — along with red and green. This light is intended mainly for situations where you need to effectively illuminate the space in front of you, but it is undesirable to use ordinary white light. Human vision is most sensitive just to blue and green shades; therefore, a relatively weak blue light flux allows a large amount of detail to be revealed. And in some situations, such lighting can be even more effective than white. For example, if at night a white lantern is pointed at a light object, then the space behind this object will be hardly noticeable due to the bright reflected light; and a weak blue light will evenly highlight both the "foreground" and the "background". But using this shade at high brightness, on the contrary, is undesirable — reflection from bright blue light will dazzle even more than from white, and even more so red. And if a blue beam, even a weak one, hits directly into the eyes, it will instantly knock out night vision, and it will take quite a long time to restore it.
Note that the choice between blue and a similar green colour depends on the specific situation: in different situations, different shades may be optimal.