How to set number of articles per page on the Blog module?
Looking at blog_page_last and node_page_default, I see that both use the persistent variable called "default_nodes_main" (which defaults to 10) for the number of nodes per page. That variable can be set on the form at /admin/content/node-settings, under "Number of posts on main page".

The above means that both the /node and /blog pages use the same value. If you want different values for both pages, you probably need to rebuild one (or both) of them as a view.

I hope that answers your question.