Most likely they are a mix of both. Some might be based on real events that have been exaggerated over time. For example, a tragic accident that happened on a Saturday night could have been turned into a horror story with added spooky elements.
Many of these stories are probably urban legends. In Pittsburgh, like in any other city, there are stories that get passed around for the sake of entertainment and to give a sense of mystery to the city. However, there could be some elements of truth in them. Maybe there were some strange occurrences that people couldn't explain at the time, and these were turned into horror stories. For instance, if there were unexplained disappearances on Saturday nights, it could lead to all sorts of wild stories being created.