It's never even explained how bloodbending blocks off other bending. All we know is that energybending somehow fixes it, but given how nonsensical a lot Korra's rules are, we shouldn't correllate that to mean that Bloodbending = energy bending. And it would certainly not make sense that a waterbender could control lightning, unless you think Lightning just turns into spiritual energy once it enters the human body.
And you can keep saying Amon controlled his own body to counter Tarrlok's bloodbending with his own, that doesn't mean you know it. I'm not even saying that it's not what happened, because it could have just as easily been that. The point is you have nothing in the show that definitively supports it.
As for why using the big move would be preferable is because it'd probably kill them (assuming they don't have magical bloodbending abilities), and then they would be forever distracted. And it's not that big a move. It's a one second charge up. His normal firebending probably takes only a fraction less time to shoot.
Well, if your objective here is to 'beat' me rather than have meaningful discussion, then whatever. Contrary to what it might look like, I don't go into debates to 'win', do it because I enjoy good insights and using them as a correction of my own blindsights, much like how I have misinterpretted the ending of Zuko's fight for all this time. If you're just doing it to be a dick about it, well...then your being a dick about it.
And it
is a bit off, so
I know! It's a tough crowd today, lol