Hi, Tom!
I have the same problem. The Blog Add-on is activated but it doesn’t show in the Customizer at all. I have the latest version of wordpress and GeneratePress. All Generate Press add-ons are activated.
What am I missing? Thanks!
UPDATE: So, I figured this one out… The Home page was set as a static page & posts page. I changed it to latest posts and voila! The Blog settings showed up.