Is there any reason we were taught in school that blood is blue in the body until it reaches oxygen and then it turns red other than our veins look blue?

I have always wondered why I was taught in school for so long and it turns out that apparently lots of other people were taught this too.

The only reason I can think if that we were raight that blood in the body is blue is because our veins look blue. But does anyone know where that myth came from or why kids are still being taught this today?