365: Day 116 - Why veins appear blue...
In humans and other hemoglobin-using creatures, oxygenated blood is a bright red in color. Deoxygenated blood is a darker shade of red, which can be seen during blood donation and when venous blood samples are taken. However, due to an optical effect caused by the way in which light penetrates through the skin, veins typically appear blue in color.
Got the info from "The Internets"
I was already in bed when I took this SP.