From b2evolution manual

This manual page is outdated. NEW PAGE: Blog ALL in b2evolution v5+.

b2evolution version 0.x & 1.x only -- doe NOT apply version 2.x and above

Blog #1 automatically aggregates all the content of all other blogs.

In B2evo 2.x & above, any blog can be set to aggregate any number of other blogs by entering a list of individual blog IDs to aggregate.

[edit] A common mistake

One of the biggest errors new users make is choosing the "Blog All" as their primary blog page. "Blog All" is a special blog that shows every post that is posted in every single blog on your site. "Blog All" is useful for many things, but it is not intended to be your primary production blog.

[edit] The prefered method

If it's just you, and only you, who will use the blog, using "Blog A" for all your content is a good choice. Recycle "Blog A" (rename it and deprecate all existing posts). If you only want one weblog you can hide "Blog B" for the time being. (Again, we don't recommend deleting it right away. To hide a blog, open the "BlogSettings" tab, click on the blog you want to change and in the "General" tab under "Default display options" uncheck "Display public blog list" and "Include in public blog list". (This appears to have changed. If you go to Blog Settings >> Features find "List of public blogs" you will find the option "Include in public blog list:" where you can display or hide your blog.) Many people start expanding their sites with multiple blogs after they get used to how b2evo works.) The "Linkblog" you can use or hide, depending on whether or not you want links in the sidebar of your "Blog A" or not.