In colour theory, light is primarily composed of three colours: blue, green, and red. White light results if each is present with equal intensity. A blue object is one that absorbs green and red light but reflects blue. (In reality, white light is obviously a continuous spectrum, but the basic idea still works)

If you shine a blue light on anything that doesn't reflect blue light, it's going to appear black. If it *does* reflect blue to some extent, you're going to see a lighter or darker shade of blue (/gray) depending on how much of the light is reflected. In the case of flesh-tones, this tends to produce some corpse-like skintones, especially on light-skinned people. Green is also likely to appear black-ish.

I think. This is physics/art, not chemistry :-)

XWA