Is it illegal for bank black households to transfer assets?

It is not illegal, but it is not recommended, because it may be enforced at that time. As long as it is enforced, if you are in China, including Hong Kong and Macao, the local law enforcement department will come to your door and all assets will be frozen. Unless you immigrate overseas directly, it will be a country with no diplomatic relations with Chinese mainland, otherwise they will still find you.