I’ve heard so many different things. It’s blue, it’s greenish, it’s dark purple. What’s the true color before it hits oxygen? Even google has conflicting answers.
Venous, deoxgenated blood is dark red. This is easily observable if you have a blood sample taken with a clear syringe. (Although there is air in the syringe, it’s not enough to oxygenate the whole sample.) Certainly not blue or green.
I should clarify, there should be almost no air in the syringe if blood is being drawn, rather than just very little.
I asked my father the cardiologist when I was ten. He said it was blue.
A duck!
He must have been joking, or taking about lobster blood.
Uh, blood often contains oxygen, so this really doesn’t make sense. What you probably mean is “What’s its true color while it’s in blood vessels?”
The answer is basically “It depends”.
Blood with a high oxygen content is bright red. This is the blood you’d find in your arteries, which take oxygenated blood to your tissues. You will find that blood from your capillaries is fairly well oxygenated, so it is a fairly bright red. Thus, a superficial cut will issue red blood.
Blood with a low oxygen content is a dark red/maroon/purple. This is the blood you’d find in your veins, which take deoxygenated blood back to your lungs. This is the color of blood you see when someone takes blood for tests; it’s also the color you see when you give blood.
The color difference comes from the electronic state of the “heme” complex that is the core of hemoglobin, the protein that binds oxygen.
Red blood cells contain hemoglobin; dissolved O2 and N2 migrate through the alveoli in your lungs into the lung capillaries where the hemoglobin binds the O2. Hemoglobin holds onto the O2 until it reaches cells. At that point, other proteins release the O2.
One of the reason carbon monoxide poisons you is that hemoglobin binds CO better than O2. CO eventually takes over all the hemoglobin, so your body is unable to uptake O2.
When hemoglobin binds O2, the absorption spectrum of the heme “relaxes” so that it reflects a lower wavelength of light (bright red). When the O2 is released, the heme reconfigures and absorbs/reflects a higher wavelength (blue/purple).
The pertinent mailbag article: http://www.straightdope.com/mailbag/mbloodrd.html
But what colour is it when nobody is looking? In a forest.
Blood always has oxygen in it. Arterial blood has (ideally) more oxygen than venous blood, but there is still some. Blood is red , bright or dark, but red, nevertheless. Of course, once outside the body for several days, it appears black, but adding water shows it as red. Here is a simple answer to: “Why is blood red?”
Just to note the the OP didn’t specify human blood.
Purple? Human blood is sometimes purple?
Blue has a higher wavelength? Don’t you mean frequency?
No! Red! ahhhhhhh!
What if the blood is on a treadmill?
You realize that about half the blood in the body has “hit oxygen” in the lungs, right?
So that half is red. Blood-red, more specifically 
So why are my veins blue?
Royalty?
Blood on a treadmill won’t echo, and no one knows why.
Read the link in post #8.
When I was a Civil War reenactor, I bled Union blue, baby.