it likely wasn't JUST the blood; it was the 'perceived severity' of your friend's injury, too. Your response was typical, actually. People are more squeamish when someone they know and like is injured because it's easier to empathize with them (you really DO feel their pain.) Plus, the symptoms you describe are ones of stress/excitement (your 'fight or flight' was out of whack); you had a vascular response to too much adrenaline, which can cause fainting, ringing or other sounds in your ears, shortness of breath....Next time tense your whole lower body; hard. It forces the blood back to your brain and stops the fainting feeling.
It's only 'about the blood' if you choose to MAKE it be about it- then the phobia is slowly hardwired into your brain and behavior. Though, truthfully, some blood squeamishness is already hardwired in. I had that happen to a nursing student while I was showing him a MACHINE console; I don't think he went on to become Amish. Just put it behind you, and realize you're one of the people that can faint, and watch out for THAT instead.