Why is blood red?
The short answer
Hemoglobin, a protein in red blood cells, carries oxygen and gives blood its red color.
How it works
Oxygen-rich hemoglobin looks brighter red, while oxygen-poor blood is darker red. Veins can look blue through skin because of how light is absorbed and reflected, not because blood is blue.
A familiar example
Blood from a cut does not change from blue to red when exposed to air; it was red already.