Give black knight steady breath?

A -atk B!Ike is still awesome. He's a rare unit that doesn't suffer much from any bane. However, if you love BK that much, go ahead and you won't regret.

If I were you, I would go ahead and sac Ike to give BK Steady Breath. I have the same dilemma, as I only have a single b!ike. I decided to keep mine for the time being until I pull another someday though, I have +def/-hp and I like his IV's, plus I don't have many 5* greens.

I have the same BIke and he still owns any map/event. I don't recommend sacrificing him unless you manage to get another one

That boon/bane combination might not be optimal, but Brave Ike is still good, and totally useable. The Black Knight is really really good with steady breath, but fury + vantage or any other set is good as well. I wouldn't sacrifice Ike, wait until you get another one to give steady breath to the BK. It is better to have 2 strong units, than just one OP unit.