Assuming both characters know something about each other, Byakuya keeps his eyes closed (which he usually does anyhow. Seriously, how does the guy get around?) and flash steps behind Itachi for his signature backstab. Senbonzakura through the heart, and the crowd boos for the lack of a decent battle.
I dunno much about Bleach but IMO for someone to call this, the person needs to have read both. My understanding is Itachi is capable of beating anyone that is capable of beating themselves (meaning if they have the capacity to kill themselve with their own attack, Itachi would generally win by just looking at them, he doesnt even need eye contact like what he did to Kurenai, the only difference is he lets them kill themselves). But tahts just me from a naruto stand point, i dont no enough about bleach to make the correct call