Welcome aboard, Walter! You must be sailing on Disney's newest ship, the Disney Fantasy, since you're asking about a port stop in St. Thomas. The Fantasy is an absolutely stunning ship with activities galore, but also many chances to simply relax and do nothing. I'm sure you will find just the right balance of fun and relaxation on the ship!
Unfortunately, Disney Cruise Line is not currently offering a Port Adventure to Blackbeard's Castle, though that would be pretty appropriate with the festive
Pirate Night you'll be experiencing onboard the Fantasy.
If you'd like to go to Blackbeard's Castle on your own, it is very easy to do, and though I haven't been myself, I understand that you'll only need about an hour to experience it all, so that will leave extra time to explore the port area in St. Thomas on your own. As you get to the port from the ship, there will be many taxis that will be able to take you to the castle. They start high, so be sure to negotiate a good price for your taxi trip there. Once there, it will cost about $14 per adult (kids are free) to go in and explore the grounds. I hear it is a beautiful view of the island from up there!
Have fun as you continue to plan your cruise, Walter. Be sure to let me know if you have any further questions before you set sail, as I'd love to help!