Not me but a friend thought (until we set her straight) that periods were blue because of the commercials that show how much the pad absorbs. She was so embarrassed when we told her that the blood "down there" is the same color. So was her face :)
A childhood belief about periods

others believed things like this
- I used to believe that menstruation fluid was blue because of the way TV commercials showed it. — matias
- I thought the blue veins in some cheeses were actually blood vessesls!— Helen
- I was horrified when my first period arrived. It wasn't blue, like in the commercials for sanitary towels.— Jane
- I used to think that blood was blue until it touched air.— paetrw
- i used to believe that people with blue or green eyes saw that certain color— mellactica
- I used to believe that north was always straight ahead. I could never face south, but I knew it was behind me!— Bruces