Designed to reduce the burden of blocking when many accounts are attacking you, or when a few accounts are attacking many people in your community. It uses the Twitter API. If you choose to share your list of blocks, your friends can subscribe to your list so that when you block an account, they block that account automatically. When blocklists are updated, the changes are propagated without your having to do anything.
Requires that you grant access to your Twitter account to the service.
If there are accounts that you blocked before you started using Block Together, and then you subscribe to a blocklist which happens to include that account, and then you stop using that blocklist, the accounts will stay blocked. It doesn't un-do any blocks you put in yourself.
Blocklists top out at 250k accounts.