Even such a simple question can be answered with two reasons:
Blue blood
Our blood is red because of hemoglobin - a complex protein that includes an organic compound with iron - heme. The main task of hemoglobin is the attachment, transfer, and release of oxygen in the body. Accordingly, when hemoglobin is without oxygen, it acquires a red-blue color. and when with oxygen, it is bright red.
And taking into account the fact that venous blood is poor in oxygen, that is why it is purple-blue
Optical illusion
Despite the bluish tint of venous blood, this is not enough for the veins to be completely blue. This is where the second reason comes into play - optical illusion.
Light is a beam of light waves that does not have any color. Each light wave has a different length. For example, violet waves reach a length of 450 nanometers, and red ones - 630. When they hit any surface, they are reflected from it and reach our eyes. Therefore, all we see is the reflection of light waves.
Red waves penetrate inside and are absorbed by hemoglobin, practically without being reflected. But blue waves are reflected almost completely. Therefore, looking at the veins, we see them blue.
For clarity, you can shine a red light on your hand. In this case, you may notice a dark line where the veins run. If you shine blue light, the veins will not be visible at all!
Among aristocrats of the 19th century, pale skin was considered a sign of nobility. They avoided the sun in every possible way and therefore did not become covered with a layer of tan from sunlight. On the pale skin of aristocrats, blue veins were more visible. But on the dark, tanned skin of commoners, there were no noticeable veins.
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