Blood from arteries are light red, while blood from veins is dark red. I don't know, but could not 'blue blood' be a term used for deoxygenated blood?
... the only people with ski so thin that the veins could be seen through it were the "noble" ones. Looking at their hands it seeemed like they had blue blood.
At least that was the explanation of "blue blood" that I got when I was a kid ;-)
I found this very detailed explanation, in a nutshell skin scatters light. https://www.imt.liu.se/edu/courses/TBMT36/pdf/blue.pdf
And this list of animals with actual blue blood, when its healthy and oxygenated not de-oxygenated: http://www.buzzfeed.com/donnad/10-animals-that-bleed-blue