If it says "Black Tie Optional" then it's appropriate for men to wear dark suits, but if it just says "Black Tie" then men are expected to wear tuxedos. Women do not have to wear long gowns in either case, and given it follows a night of theater, I would expect that many or most women will not be in long gowns.
It is VERY unusual today for Opening Nights to be formal attire.
CZJ at opening night party for A Little Night Music, Dec 13, 2009.