One of the neat things about AO3 is that you can, essentially, add your own descriptive categories on the fly. If you are publishing a story that you think needs to be tagged as darkfic, you can just enter that as a tag, and it will be displayed and become searchable. There's supposed to be a way (I think) to tag existing stories with additional flags that you, as a reader, think they need... but I don't know how to do that.