BuddyPress.org

global $blog_id;
echo $blog_id;

Or
In WordPress 3.1, your $blog_id can be found by navigating to the “Network Admin > Sites” page and hovering over the blog in question. You should see a link that resembles this:

http://example.com/wp-admin/network/site-info.php?id=1

Your $blog_id, in this instance, would be 1.

For WordPress 3.0, the $blog_id can be found in the “ID” column of the “Super Admin > Blogs” page.