if I tattooed my blood type on my arm, would paramedics ever use it?

No. There is no way to verify that the information tattooed on you is correct and the consequences of being wrong are too severe.

Can you imagine the lawsuit if you had "A pos" tattooed on you, they elected to give you A pos blood based solely on this, and it turns out you have type B pos and die of a transfusion reaction? What would the doctor's defense be? "Well I figured it was probably right so we didn't bother to check." We wouldn't take your word for it if you told us you were A pos without a lab test. We don't even trust other hospitals' labs much less a random tattoo. You will NEVER get typed blood without a type and crossmatch beforehand. In an emergency, you'll get O neg until a T&C is done.

Save the tattoo money for something you'd really like inked. Don't waste it on a blood type.

A blood transfusion is given only AFTER all tests show it is compatible. So in an emergency, IV fluids or plasma are used first. Bleeding needs to be stopped and that is what is normally done. In a remote circumstance, it means that O negative would be used.

You can tattoo anything, but this is certainly not the whole picture. Some 40 different items are checked during the cross match. You would need a lot of information on your skin for this to be useful.

No, they would not rely just on that.

Before ANY transfusion, the doctors must take a small sample of your blood and do standard blood type tests on it.

not until it's cross checked