Blood inside a vein is not blue. Haemoglobin makes oxygen-rich blood a brighter red, while venous blood with less oxygen is a darker red. The blue-green colour seen through skin is an appearance produced after light enters tissue, where it is absorbed and scattered.
When white light reaches the wrist, some returns from shallow layers and some travels deeper. Different wavelengths penetrate, scatter and are absorbed by blood to different degrees. Above a vein, the returning red component is reduced more strongly relative to the surrounding skin. The visual system compares the vessel region with nearby skin, so the darker strip with relatively less red may be perceived as blue or green. A 1996 study using tissue models, measurements of human skin and simulations of light transport showed that vessel depth, diameter and the optical properties of skin all influence the apparent colour.
It is therefore wrong to conclude that blue-looking veins contain blue, deoxygenated blood. Venous blood drawn from the body remains dark red. Skin tone, lighting and vessel depth can make similar vessels look blue, green or barely visible. Colour here is not a label carried by the object alone; it emerges from light, tissue and visual comparison together.
https://pubmed.ncbi.nlm.nih.gov/21085227/
https://opg.optica.org/abstract.cfm?uri=ao-35-7-1151
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