In Postman (for those using filters in the URL), for me, this has worked:

localhost:8080/api/blogs?populate=*&filters[categories][uid][$eq]=react

I have a blogs content-type and a blog-categories content-type with a many-to-many relationship. A blog can have several categories, and a category can have several posts. So, if I want to filter blogs that belong to a specific category, I do it as I described above. 👆