Well, without having read the 'the black pirate ship christmas story' precisely, I can only guess. It might involve a group of pirates on a black ship. At Christmas, they could be facing some sort of adventure. For example, they might sail to an island where there are some Christmas - related mysteries. They could meet strange creatures or find a hidden treasure that has something to do with Christmas. Or perhaps they get visited by a Christmas spirit who tries to teach them about kindness and sharing, which is quite different from their usual pirate ways.
Since I'm not familiar with this particular 'the black pirate ship christmas story', here's another possible scenario. The black pirate ship could be on a journey during Christmas. The pirates on board may be a bit grumpy about missing out on the usual Christmas celebrations on land. However, as they sail, they come across a shipwreck. When they explore it, they find a chest full of Christmas presents meant for a nearby village. At first, they think about keeping it for themselves, but then they remember the true meaning of Christmas (maybe from their past memories) and decide to deliver the presents to the village, thus having a very different kind of Christmas experience.