The Great Firewall does a lot more than that. It blocks new IPs by default (some are later re-enabled after being 'assessed' in some way).

It does some kind of automated crawling and blocking.

It has manual (as in human) blockers, who read content on Weibo and remove it.

It's still horribly crude, and fairly straight forward to route around but it's more complex than a simple static dns blacklist.