Actually having your blood type on your helmet would be a good idea as long as it was accurate.
As you probably know rally cars often have the names of the driver and navigator painted on the side of the car with their respective blood types.

The problem with the guys in the Army was that they often got their blood type wrong and the tattoo artist just did as he was told.
Young guys :rolleyes:... What would you expect?

No one ever, EVER will give you a blood transfusion without first determining your blood type THEMSELVES. It only takes a few minutes. If there is no time for that, you get O- blood. Period.

No one will ever transfuse blood based on what you've written on your helmet, your car's roof or tattooed on your forehead. That's just to impress girls.

The nice WWII vet down the street has his blood type tattooed under his left armpit. He claims it saved his life in the Ardennes in December of '44. He still has the slightest German accent. He says he got his accent from his parents growing up on the Iron Range, but I have my doubts. He says that if pasting your blood type on your helmet or under your arm gives you comfort, you should do it.

my wife and I found out we were the same blood type when we went in to get married, actually donated a few units for her at various times in Texas, they charge you like $500 here in california

Have never heard of the O neg to O pos switch thing