I think you’re confusing the symbol with reality. They used to show unoxygenated blood as blue and oxygenated blood as red on the chart so you could visualize how it passes through the system and what occurs. I don’t recall anyone telling me my blood is physically, visually blue.

I was told it was purple.

We werent

I know the mitochondria is the powerhouse of the cell.

I think about this every time I get blood drawn... We were literally told that it's because our veins look blue/green.

Not just millennials, I think a lot of science teachers had no love of the subject so just answered off the cuff or directly from the textbook Which is really sad as it leads to a lot of misinformation

I never once learned that in school. I heard it as a rumor from kids. Never once learned in school though.

Because blood doesn't turn red until it hits oxygen. Aka cut open. Like blue crabs turn red when cooked.

Elder millennial. I was not taught this?

I don't remember being taught this by a teacher. It's always been something the kids say to other kids for a "wow moment".