The guid in the blog add-a-blog url is simply that of the container entity.  So if it's a personal blog it would be the guid of the logged in user.  If it's a group blog it would be the guid of the group.

Ah I see now, so how is the guid for the container generated?

GUID is created automatically to the database when you create an entity. So when you make a "Add something" button/link, you just do this:

'href' => "my_plugin/add/{$container->guid}",

For example: https://github.com/Elgg/Elgg/blob/1.10/mod/blog/views/default/blog/group_module.php#L33

There's also shortcuts like this: https://github.com/Elgg/Elgg/blob/1.10/mod/blog/lib/blog.php#L102

If you're asking how any entities guid is generated at the base level it's an auto increment of the guid column of the entities table.

Interesting, ok got it thanks!