In order to create a Mara cms blog, the main requirement is for pages to use a blogging theme. This theme will call the blog plugin code, and will insert the blog-specific text elements into all such pages.
If the entire website will be in blog format, then it is a relatively simple matter to declare a blog-format theme as the default in siteini.php. All pages will then behave as blog articles, regardless of whether they were created online or uploaded.
If you wish to make a subsection of your site into a blog, but to retain conventional website behaviour for your main pages, then the way to do this is to set the cms_theme meta tag in the head section of blog pages to reference your blog theme. Leave the site's default theme as a conventional type.
To save the need to modify the page head section by hand, there is a BlogArticle template available under File..New, which if selected will include this setting in your new page.
It will generally be more convenient to make your subsection blog a subdirectory of the site's filesystem structure. This way it will have its own index page, and the catalog of articles (TOC) will appear here instead of on the main site's homepage.