The models in the blog app (custom or not) need not inherit from the Page model at all.
You have your blog app set up, its own models, views, and urlpatterns. Those urlpatterns will define the URLs for the different parts of the blog app.
The way things are integrated with pages, is that via the admin, you can add a page (of any Page type, but usually a RichTextPage), and give it the same URL as one the URLs defined by the urlpatterns in your blog app.
So in the blog app we have the urlpattern for /blog/ which loads the blog list view. Then in the admin we make a page and give it the URL "blog" (the page urls don't include leading/trailing slashes). Your blog app's view will be loaded, and the templates it loads will also have access to the blog page you created in the admin, for things like page title, keywords, etc.
Hope that clears things up.