The china firewall is complex. In this case, github.com isn't actually blocked in the strict sense. Its just that the DNS for some or all of China's DNS servers doesn't have the correct records. If it were truly blocked, even using the correct IP won't get you there.
This could have happened if github.com changed its DNS records and the China DNSs didn't pick it up. This happens sometimes. The only solution I have found is to re-trigger the DNS change so that next time the China DNSs might pick it up. Otherwise, its anyone's guess when the China DNSs might get corrected. How do you re-trigger the change notification from your primary DNS? I'm not expert enough to know how to do this non-destructively. When I've had these problems in the past, I've had to drop the records and re-add them.
This could have happened if github.com changed its DNS records and the China DNSs didn't pick it up. This happens sometimes. The only solution I have found is to re-trigger the DNS change so that next time the China DNSs might pick it up. Otherwise, its anyone's guess when the China DNSs might get corrected. How do you re-trigger the change notification from your primary DNS? I'm not expert enough to know how to do this non-destructively. When I've had these problems in the past, I've had to drop the records and re-add them.