Why a vein can look blue through skin
Blood is never blue inside a healthy body. Even blood with less oxygen is a deep red; oxygen changes how hemoglobin, the oxygen-carrying protein in red blood cells, absorbs light, but it does not turn blood blue.
The color you see at your wrist is made by light traveling through skin. Sunlight and indoor white light contain many colors. Skin scatters some of that light back toward your eyes, while other light travels deeper and is absorbed. A vein changes that balance: blood beneath it absorbs much of the light that reaches it, especially red light. The light returning from the skin above the vein therefore contains relatively less red, so it can look bluish beside nearby skin.
The vein is part of the effect—not a blue object
Blue light does not simply bounce off a blue vein. The appearance comes from the combined effects of blood absorbing light, skin scattering it, and the vein’s depth and size. Because these conditions vary, the same vein can look blue, greenish, or barely visible under different lighting or through different skin.
That also explains why a vein looks red when blood is exposed to view: without the overlying skin to filter and scatter light, the blood’s own color is visible. The blue is an optical effect of seeing red blood through tissue, not the blood’s actual color.
