John, your black velvet jacket outfit will be nicely dressy, considering it's a Christmas cruise. A dark-colored suit will also work just fine for formal nights. We did a southern Caribbean cruise on Golden Princess this past February and many people dressed up rather fancy for formal nights - from what you describe, you will probably be exactly right.
A few tips for anytime dining: the most popular times seemed to be from 7 or 7:30 on. We found that the dining room on the lower deck was less busy - probably because it was farther away from most cabins and the evening action (though riding the elevator should have made that a moot point!)
If you like beef and the Caribbean Princess has a special steakhouse restaurant, I highly recommend it. At least on our ship, the steak was excellent, better than the ones in the main restaurant, and service was tops.
Have a wonderful cruise and a very merry Christmas!