Human blood is colored due to the red blood cells containing hemoglobin. However, hemoglobin is an iron-rich molecule that can be bright red or darker, dull red. However, when looking at the veins, we see that they are blue.
This is explained by the fact that different colors have different wavelengths. For this reason, they are reflected and absorbed in different ways when they come into contact with the skin. Red, for example, has a long wavelength, so it will easily pass through the skin, but blue, which has a shorter length, on the contrary, will be reflected.