We know that oxygenated blood is red because we can see it with our eyes! Scientifically proving something requires repeated observation. The claim is that oxygenated blood is always red; if we consistently see that oxygen makes the blood increasingly red, it is a strong indication that oxygen is the determining factor in making blood red. The blood is never actually blue, it just appears blue when you look at it through your skin because of the way light penetrates your skin. Deoxygenated blood is a dark shade of red and as it becomes more oxygeneted it becomes a lighter shade of red. To be sure deoxygenated blood is not blue you could imagine setting up and experiment where you put blood into a chamber that is devoid of oxygen, if it does not turn blue when there is no oxygen, then it would support the claim that blood is always red. I hope this is helpful!