Yes, I did. I didn't notice the problem until many turns after I took Fargan's Keep — I had a leader that I decided I didn't need for the moment and wanted to park in my city with the highest unrest.
I also decided to test razing the Keep, and that fixed Blackstead too. It's definitely linked to Fargan's Keep.
Thanks for the follow-up, Davrovana. It's good to check these things.
Blackstead is my most developed and important Fortress city, so this issue pains me.