BuddyPress.org

Whatever theme you want to be the default for your blogs needs to be put in the ‘default’ folder in the wp-content > themes.

If you want to use the buddypress home theme for the blogs copy it (with your customizations) into the default folder. The folder needs to stay named Default. When WPMU/BP creates a blog, it will use whatever the Default theme is.

All the best to you!