Imagine you had a massive torch and you took it close to a black hole's surface and directed the torch's rays towards the black hole. The rays that reached the black hole's surface would disappear. They wouldn't travel anywhere else. So, no one can receive them and the black hole would remain invisible.

By the way, I hope it is now clear why such an object is called a black hole. It is black because even light cannot escape its surface. So, no one can see it. It is a hole because once something reaches its surface, it can never get out of there.