IMO, a 55gal, assuming we're talking about standard tank dimensions, would be a better tank then a 65gal, because of the extra length, vs. the extra depth of the 65gal. The Butterfly would have more room to swim.
Not that a 65gal is the worst tank for a Blackback. They are some of the smaller Butterflies, just perhaps, some years down the line, a bigger tank for him would be a good thing.
BTW, he's not going in any type of reef, right?