The "block" function on Twitter is supposed to keep users safe from online harassment by preventing someone from viewing your profile and content. But trolls can just create a new account if they want to. So the block button has really just become more of a message than a useful resource—a way of saying "FU" to someone. But it often conveys something deeper: hurt, or anger, or both.
This might be why people seem to enjoy getting blocked by a celebrity. Because it means you got through to them somehow. And if there's one thing most people seem to want in this world, it's to get celebrities to pay attention to them. Even if that attention is negative.