According to UCSB ScienceLine, human blood is always red even before being oxygenated. It is a myth that blood is blue while it is in the veins. In reality, it is just a brighter red when it is carrying oxygen and a darker red when it is depleted of oxygen.
Mental Floss elaborates on this myth, suggesting that it stems from the blue appearance of human veins as seen through skin. This is caused by the absorption of different wavelengths. Blue light tends to not penetrate as deeply into the body, and it is reflected back out to be seen by the eye, giving deep veins their distinctive blue color when seen through skin.