Heavy Blade on Black Knight

It could be a "crazie failure." On a serious note, BB's atk isn't high enough to utilize Heavy Blade, at least not yet. If you're determined to do so, maybe a Delthea can make it work.

If you could boon his Str I'd say it would be amazing, but you can't boon or bane him so I'd avoid it. Too many other units in arena and high level maps will have higher atk then him, making it useless.

Besides, Steady Stance, Steady Breath, and Fury are all better on him. And if you can get a Wrath on his B slot then you won't have as much usefulness in Heavy Blade anyways.