My impression (and it was definitely spelled out in writing on Silversea) is that if it is formal night, you are expected to be dressed according to code in all public venues except the restaurant that might allow more casual dining.
ChatKat, I think many of us ladies enjoy seeing people all dressed up now and again. Men, are you listening? Of course, while the traditional formal wear for men has not changed over the years, we females no longer have to wear those corsets, girdles, whalebones, long gloves, and heavy dresses of yesteryear cruises. We can just put on that black dress you talked about and dress it up with a little of this and a little of that.
My daughter, 22, is going with me on VJ7, and she gets really excited about "formal" nights. Just today, she went out and bought not a couple of "formal gowns" but a couple of skirts that she thinks will fit the bill. Young people have a different idea these days about what is "formal". She will have a great time thinking she is "dressing up to the NINES", so to speak, because her world has been so casual. I think to her that may mean putting on a pair of earrings that hang longer than 1 1/2 inches. I am not going to stress about it, because our fellow lct friends on this cruise are mostly parents of young adults and will understand!