BuddyPress.org

You need to get the owner of the blog, then get the avatar for the owner. Something like:

$blog = new BP_Blogs_Blog($blog_id);

$av = bp_core_get_avatar($blog->user_id);

bp_core_get_avatar() has lots of options. Or $user = new BP_Core_User($blog->user_id) for even more stuff related to the user who owns the blog.