The command is "add notranslate modules <module> -both". Use it before design library read-in. Often the term "black boxing" means using "add notranslate modules"
It is useful for memories, analog circuits, and "non-synthesizeable" code. If you don't "add notranslate module" to large memories then you can have problems with excessive run-time and server memory.
It is also useful to check what is black boxed after read-in with the command "report black box".
There are more examples, "troubleshooting info", etc. if you search http://support.cadence.com