According to one of the leading experts on the human circulatory system, blood flowing through veins is blue.
I’m not going to mention any names. All I’ll say is this: A person I know visited a major research center last year and saw a demonstration of organ removal and some other experimental stuff. A person also visiting asked the famous high-level researcher doing this work if blood was ever blue. What he said was not recorded in detail, but it was very much like this statement I found on the Internet:
… human blood is red as soon as it is oxygenated. Blue blood flows through veins back to the heart and lungs…..
[source: Some Guy on Yahoo Answers]
My friend was disturbed by this, as s/he had been teaching high school students for years that blood is not blue. Her understanding of the situation was that people thought blood was blue because standard anatomical drawings and models depict arteries as red and veins as blue, and because if you look at your veins they are blue. Obviously veins are not clear, but if you don’t think that out you might assume that you were seeing blue blood.