Update: Twitter has reversed the following changes after a significant number of users voiced their outrage on the service. "We believe this is not ideal, largely due to the retaliation against blocking users by blocked users (and sometimes their friends) that often occurs," the company said.
An update to Twitter's blocking policy changes exactly how blocked users can interact with the users who have blocked them. Bear with us for a moment as we step through this.
Previously, if you blocked another user, that person could no longer follow your account, although they could still see your tweets (assuming your account was public, as most Twitter accounts apparently are). Twitter wouldn't tell other users that they'd been blocked, although they could figure it out if they tried to follow you and couldn't.
Now, though, any user you block can follow you and view your tweets directly in their timeline. They can also favorite, retweet and @-reply to your account—you just won't see any of that activity in your timeline or your "connect" tab. You also won't see anyone else who RTs, favorites or follows blocked users, and you won't see blocked users among your followers.
In effect, blocking another user "mutes" them for you, but changes nothing for them.
A Twitter spokesperson told me that updating the blocking policy is intended to reduce the antagonism some people express when they've been blocked. Sometimes users who found out they'd been blocked would retaliate by getting his or her friends or other accounts to tweet harassing comments at the blocker.
Twitter added that the move should also remind people that everything they tweet is public (again, unless they have a protected account). That means anything you post publicly on Twitter can be seen by everyone with an Internet connection.
.@twitter "Block" and "Ignore" are not interchangeable terms. It's why I have locks as well as curtains.— Rachel Edidin (@RaeBeta) December 12, 2013
As you might expect, many users are upset with the changes. Whenever a company tweaks privacy policies to make data more publicly accessible, the Internet tends to balk.
However, blocking someone on Twitter has never done anything more than add an extra step for them to read and interact with your tweets; the new update effectively mutes aggressive or annoying tweeters, and hopefully ends some of the hostility toward blockers.
If you really want to keep the world at large from viewing your tweets, simply set your account to private so only people you approve will be able to view your timeline.