What color is deoxygenated blood inside your veins—is it actually blue?

No. Deoxygenated blood is dark red, never blue. Blood is red at every oxygen level because of hemoglobin, the iron-containing protein in red blood cells. Oxygen-rich blood is a bright, vivid red; oxygen-poor blood is a darker, more muted red (sometimes described as almost maroon). The difference is a shift in shade, not a change to a different color. Anyone who has had blood drawn from a vein has seen this directly: the blood that comes out is dark red, not blue.

So why do veins look blue or blue-green through the skin? It is an optical effect, not the color of the blood. Three things combine:

1. Wavelength-dependent light behavior in skin. White light hits your skin and has to travel down to a vein and back out for you to see it. Long-wavelength red light penetrates deeper into tissue and reaches the vein, where the dark venous blood absorbs much of it. Shorter-wavelength blue light is mostly scattered back out by the superficial layers of skin before it ever reaches the vein. The net result is that the light returning to your eye from over a vein is relatively depleted in red and relatively richer in blue. (This is the same family of wavelength-selective scattering that reddens the setting sun.)

2. Contrast with surrounding skin. The detailed physics was worked out in a 1996 peer-reviewed study by Kienle et al. in Applied Optics ("Why do veins appear blue? A new look at an old question"). They showed the vein need not actually send back more blue than red light in absolute terms; rather, the vein reflects less red light than the surrounding skin, and human color perception is relative—your visual system judges the vein against its surroundings, so the comparatively lower red makes it read as blue. Vessel depth and diameter also matter: deeper or wider vessels tend to look more blue/green.

3. The textbook convention. Anatomy diagrams color veins blue and arteries red purely as a labeling convention to distinguish them. Many people absorb that as if it were literal, reinforcing the myth.

The "blue blood" idea is therefore a double misconception: the blood is dark red, and the bluish appearance of veins is a trick of how skin scatters and absorbs light, amplified by relative color perception. (Note: some invertebrates such as horseshoe crabs and many mollusks genuinely do have blue blood, but that is because they use copper-based hemocyanin instead of iron-based hemoglobin—humans never do.)

Sources:

• https://www.medicalnewstoday.com/articles/321442
• https://www.wtamu.edu/~cbaird/sq/2012/12/18/why-are-veins-blue/
• https://www.livescience.com/why-veins-blue-arteries-red
• https://opg.optica.org/ao/abstract.cfm?uri=ao-35-7-1151
• https://www.researchgate.net/publication/47812517_Why_do_veins_appear_blue_A_new_look_at_an_old_question