iMessage isn't blocked in China because it doesn't need to be. China passed a law forcing Apple to put all Chinese users' iCloud data in a Chinese datacenter. Since Apple allows for account recovery, they store account keys in that datacenter. Anyone with privileged access to certain production resources in iCloud China should be able to access basically all Chinese users' data.

The one exception is apparently Apple's keychain as per https://blog.cryptographyengineering.com/2018/01/16/icloud-i.... But, if the hardware used to enforce the security is different in China, even that could be cracked.