Do you mean a blog directory which lists all blogs in the BP install? You must enable multisite first – create a network. Then:
1. Go to dashboard > Super Admin > Options – Registration Setting – Allow site creation
2. Go to dashboard BuddyPress > Component Setup – Blog Tracking enable
3. For users’ blogs to be listed in blogs directory, Settings > Privacy must be public.
Or do you mean just the “blog” section of the main site?