I found the majority of men do not were tux on formal nights so I would not classify a formal night as black tie. You do see men in tux but the majority of men are in suits.
Historically, formal evenings aboard ship were strict "black tie" events. In the 1970's, the "mass market" cruise lines started allowing dark business suits as a substitute, in the same manner that military and naval uniform regulations provide substitute uniforms for personnel who do not own the "black tie" uniforms, because many gentlemen did not own dinner jackets or tuxedos, but the ship's officers continue to wear "black tie" uniforms. Formal still means "black tie" according to social etiquette.