The color of blood

Many people remember being taught at school that the blood inside the human veins is colored blue, and only turns red when it comes into contact with air. The fact that it looks that way when checking is out on your arms, legs etc appears to only reinforce this idea.

In fact it's not true, and this is coming as a big surprise to many people, who in turn have been telling their children this is the case and so perpetuating the myth. It's red because the way it carries oxygen around the body is in the form of hemoglobin, which is rich in iron and gives it the red shade.

There is a clue this is the case without having to conduct some elaborate experiment. When blood is drawn into a tube via a syringe, it usually goes into a vacuum in the tube, not air. This prevents contamination, yet it's still always red.

Some non-humans are different

Spock from Star Trek has green blood. Back down on earth, there are creatures which really do have blue blood, but the reasons for it must first be understood before pointing them out. As explained, the red color comes from the way oxygen is carried around using hemoglobin. There are forms of life which don't use hemoglobin to do this, but still need to carry the oxygen around. They use the protein hemocyanin, which is colored blue. Examples of this are the horseshoe crab an octopus.

Myths such as this aren't really Mandela Effects, although some might argue the opposite. They are MMDE's, however, because it's a false memory which a large group believe. You might even find the origins of this in old text books or biology documentaries, but no-one credible in the medical profession would argue for it nowadays.