My question might sound too naive.
When would the Null0 interface act as a black hole and when for prevention of loops.
Eg:- I know in case of Enhanced Interior Gateway Routing Protocol (EIGRP), for instance, always creates a route to the Null0 interface when it summarizes a group of routes. This is basically for prevention of loop.
Similarly if I want to deny access to a particular IP or Network i can poing it to a null 0 to discard the packet.