How to add filters to a custom blog feed?

Title says it all. I want to know how I can add filters to a custom blog feed…

Also, I tried to solve this by using a blog-post-dataset instead of a custom feed widget, but this would not let me filter by category as the categories are not included in the dataset / database.

I have this same problem/question. Any answers out there for how to do this in the new Blog? It worked easily and great in the old blog.