Although it is acceptable to wear a dark suit on formal nights I would advise investing in a dinner suit. I have not sailed on the Black Prince but Fred Olsen does provide for a more traditional style of cruising and dressing up is part of that experience. Dinner suits can be bought at bargain prices these days and if you get hooked on cruising like I have then your dinner suit will get plenty of wear! If you do not like the idea of formal evenings and dressing for the occasion then there are cruise lines that offer a more casual style of cruising but for me nothing can compare with the elegance of a formal night when ladies can put on their finest gowns or cocktail dresses and the gentlemen can complement them by wearing a dinner suit.
I hope you have a wonderful cruise.
Most men will probably be wearing a dinner jacket for the formal evenings but a dark suit would be prefectly acceptable.