If we consider a common understanding, 'shaft' can refer to different things like a long, narrow part or a passage. In some novels, if there is a description of a shaft as black, it might be for creating a certain atmosphere, perhaps a sense of mystery or gloom. However, again, without knowing the particular novel, it's just speculation.
I'm not sure specifically as there could be many novels with 'riptide' in the title. It might be about a powerful and dangerous ocean current (riptide) and perhaps a story of adventure, survival, or mystery related to it.
Shaft visual novel could be a term for a visual novel that has some connection to Shaft. Shaft has created many visually striking anime series. In the context of a visual novel, it might imply a certain aesthetic quality similar to what Shaft brings to its anime works, like detailed character designs and elaborate backgrounds.
I don't know as I haven't read this 'riptide novel'. There could be a brave adventurer, a local fisherman who knows the waters well, or a young person trying to prove themselves.
One aspect to consider is the setting of the castle. Castles often have a rich history and many hidden areas. In this fanfic, the 'caught in the riptide' part might mean that the character is being swept away by events that are connected to the castle's past or some mystery it holds. It could also involve other characters in the castle who are either helping or hindering the protagonist.
I'm not entirely sure specifically what 'Shaft Pulp Fiction' is about as it's not a widely known common term. It could potentially be a creative or niche concept, perhaps a mash - up of the movie 'Shaft' and 'Pulp Fiction' in someone's unique artistic vision.