Veins look blue mainly because of how light interacts with the skin, not because the blood itself is blue. Blood is always red, but when light hits the skin, red light goes deeper and is absorbed, while blue light is reflected back to the eyes, making veins appear blue on the surface.
Skin acts like a filter. Shorter wavelengths of light, such as blue, scatter more easily in the upper layers of the skin, while longer red wavelengths reach deeper tissues. When red light is absorbed by blood in the veins, the remaining reflected light is mostly blue, creating a visual illusion.
Vein color can vary depending on skin thickness, skin tone, and how close veins are to the surface. Veins may look more noticeable with aging, weight loss, exercise, heat exposure, or dehydration, all of which can make veins sit closer to the skin or appear more prominent.
Blue-looking veins are usually normal and harmless. However, if veins suddenly become painful, swollen, or change shape or color, medical evaluation is important. Avoid self-treating vein changes, and consider seeing a dermatologist or vascular specialist for persistent or concerning symptoms.