While working with my most recent plugin, I had an issue of finding all the admins of a blog on the wpmu network. after a couple of minutes, I implemented a small hack and It worked like charm.
So here you go.
Please Note, I have used User level, which is backward compatible and is there because of the legacy plugins support(according to codex), But this hack solves the problem to iterate through the Roles/capabilities.
So here is the code.

function get_admin_users_for_blog($blog_id) {
global $wpdb;
$key="wp_".$blog_id."_user_level";
return $wpdb->get_results($wpdb->prepare("SELECT user_id from $wpdb->usermeta AS um WHERE um.meta_key ='". $key."' AND um.meta_value=10"));
}

//and now call it

global $current_blog;

$admins=get_admin_users_for_blog($current_blog->blog_id);

//now you have $admins as an array of object, Just Iterate over it and do some stuff, well, I will just print out  

foreach($admins as $admin)

echo "ID:".$admin->ID;

///hmm you can use it for more productive reasons  

That’s the shortest code I could produce, I know, The better way will be to loop through the capabilities, but hacks are for you know