Why do i go faint from blood tests or at the start of a tattoo?

It is called vasovagal syncope, a neural reflex in certain situation which is blood draw or tattoo procedure in your case. You are fainted because blood pressure drops with low heart rate and when you are laid down, BP goes up and you recover. You just have to avoid those situation or develop tolerance; it will be safer if you have those done in supine position or at least legs up at heart level.