Is human blood blue or red?

It is not the blood but the veins that "look" blue, and even then it is not the actual color of the vessel. The difference is in the way colored light moves through the tissues, and how the eye perceives those colors of light.

The oxidation state of the iron in the hemoglobin determines it's color; when the blood is oxygenated, the iron's oxidation state changes, changing the color of the blood from dark red to light red. It is never blue.

The changes in blood coloration relate to the respiratory pigment, hemoglobin. Hemoglobin is a topologically complex molecule that very readily binds to oxygen. Once the molecule is fully saturated, it reflects in the crimson red spectrum, and therefore fully oxygenated blood appears to be bright red.

After the hemoglobin molecule gives up its oxygen to tissue that needs it, the molecule becomes much less reflective, and thus deoxygenated blood appears very dark red to purple. But blue is used in charts and diagrams to contrast against red, red being the arterial system bearing O2 and blue being the "deoxygenated" blood in the venous system.

Human blood is always red. It can have different hues of red, but it is still red.