The Balls on Cunard ships take place in the Queens Room. If you don't go there, you won't even realize a Ball is going on, so don't sweat it. They do decorate the Queens Room for the themes of the Balls and some people do go a bit overboard on Ascot Night but it's usually a small number who go all out.
I do love formal nights on Cunard and wear full length evening gowns for each one. Several of my gowns are black, so the Black and White Ball is easy. The men all look terribly dashing in classic black tuxedos!