I'm not sure specifically what that particular 'giant black dildo story' is about as the title is rather unclear without more context. It could potentially be a fictional or perhaps a very strange and specific anecdote within a certain community or narrative.
Black feathers are also quite interesting. They can be used in art projects or as part of costumes for their unique color. A black umbrella can be both practical and stylish. And black - painted wooden furniture can add a touch of elegance and sophistication to a room.
Historical stories are another option. You can learn about different cultures and important events from the past, such as the story of how the Great Wall was built in China or the story of the Roman Empire's rise and fall.
It could be normal if you both have had open conversations about sexual preferences and fantasies. If you are okay with her exploring her sexuality in this way and she feels comfortable with you being aware, then it can be a normal part of your relationship dynamic. But if it makes you feel uneasy or if there hasn't been proper communication, it might not be a healthy situation.
The significance of 'the giant black dildo story' is very much in question. If it is a story that was created to make a statement about sexuality, power, or some other complex concept, then it could have significance within the framework of those discussions. But if it's just a silly or shock - value story, then it may have no significance at all, other than being a strange and somewhat off - putting title.
Since I don't know the exact 'black thunder novel', I can't say for sure. But generally, if it's a fantasy 'black thunder novel', the plot might involve a powerful black thunder that has some kind of magic or significance in the story's world.