Unfortunately, this page isn't terribly helpful. It cross-references MultiBlog. That isn't very helpful either.

There is fairly detailed information. It just isn't here. It's over in the designer section (why?), here.

This is a summary:

Where to configure it

Configure Multiblog ("MB", aka OtherBlog or "OB") from either the Preferences > Plugins in your admin interface or in your template code directly.

Attributes of blog_ids, include_blogs and exclude_blogs

To pull or leave out content from other blogs:

1. Use a comma-delimited list of blog ids;
2. Use "all" for all blogs

Examples

    <mt:MultiBlog include_blogs="1,5,8">
       <mt:Entries lastn="10">
          <p><mt:EntryTitle></p>
       </mt:Entries>
       <mt:Comments lastn="10">
          <p><mt:CommentBody></p>
       </mt:Entries>
    </mt:MultiBlog>


    <mt:MultiBlog include_blogs="all">
       <mt:Entries lastn="10">
          <p><mt:EntryTitle></p>
       </mt:Entries>
       <mt:Comments lastn="10">
          <p><mt:CommentBody></p>
       </mt:Entries>
    </mt:MultiBlog>


    <mt:MultiBlog blog_ids="1,5,8">
       <mt:Entries lastn="10">
          <p><mt:EntryTitle></p>
       </mt:Entries>
       <mt:Comments lastn="10">
          <p><mt:CommentBody></p>
       </mt:Entries>
    </mt:MultiBlog>

    <mt:MultiBlog exclude_blogs="1,5,8">
       <mt:Entries lastn="10">
          <p><mt:EntryTitle></p>
       </mt:Entries>
       <mt:Comments lastn="10">
          <p><mt:CommentBody></p>
       </mt:Entries>
    </mt:MultiBlog>

    <mt:MultiBlog blog_ids="all">
       <mt:Entries lastn="10">
          <p><mt:EntryTitle></p>
       </mt:Entries>
       <mt:Comments lastn="10">
          <p><mt:CommentBody></p>
       </mt:Entries>
    </mt:MultiBlog>

MB-Enabled Template Tags

These tags support the MB/OB syntax:

• mt:entries
• mt:comments
• mt:categories
• mt:pings
• mt:assets
• mt:tags
• mt:tagsearchlink
• mt:tagrank
• mt:authors
• mt:blogs
• mt:blogcategorycount
• mt:blogentrycount
• mt:blogcommentcount
• mt:blogpingcount

Important Note

MT:Entries also supports the blog_ids attribute. But doing

    <mt:MultiBlog include_blogs="3">
        <mt:Entries category="foozball">
            some yada yada
        </mt:Entries>
    </mt:MultiBlog>

is not the same thing as doing

    <mt:Entries blog_ids="3" category="foozball">
        some yada yada
    </mt:Entries>

The second expects that even if the blog with id No 3 does have a category called foozball, that very same category must also exist in the current blog.

So if you have this:

    BlogId 1, categories: foo, bar
    BlogId 3, categories: foozball, barbarosa

then the second example will fail, because foozball isn't a category in BlogId1 where you're invoking MB/OB.

Real subtle, that. Like 10 hours of hair-pulling subtle.

And for that very reason, it also won't work in a category archive.