Not your screen monkey (he/him)
I agree with @Mad_Jack's reasoning. You get a bonus on an attack when you expend the blood boon. It's not for the round, it's not for an action. It's an, as in one, attack, even if you're allowed to get more than one attack because of the nature of the spell whether it's eldritch blast, scorching ray, or a twinned spell.And what about multitarget spell - like EB where is Attack, so can be used, but only for first, or all of them?
And what about twined attack? there I can say - I twin spell with all of his effects.