Somehow, a category with the slug “blog” was created so the page I was seeing was the archive for that category, not the blog page itself…
That’s why I couldn’t see the changes I did in the customizer.
A post was assigned to that category and that was the only post showing up. I don’t know how that category was created, though. I don’t remember doing such thing. I couldn’t even delete it. I’ve had to change the slug.