Instead of worrying about the many, many steps that the computer might have to go though to perform that task, you just need to remember the name of the subroutine. Whenever you want your program to perform the task, you just call the subroutine.
Subroutines are a major tool for dealing with complexity.
Of course, a black box that has no way of interacting with the rest of the world would be pretty useless. A black box needs some kind of interface with the rest of the world, which allows some interaction between what's inside the box and what's outside. A physical black box might have buttons on the outside that you can push, dials that you can set, and slots that can be used for passing information back and forth. The interface of a black box should be fairly straightforward, well-defined, and easy to understand. The same goes for a VCR, although if stories about how hard people find it to set the time on a VCR are true, maybe the VCR violates the simple interface rule.