Black-box obfuscation is when you want to keep a secret but still let other people use what you made. It's like having a toy that you don't want anyone else to know how it works, but you still show them how to play with it.
The way to do it is by making your toy hard to understand. You make it so no one can see inside the box that holds the toy, it's like a secret room that only you know how to get into. You can paint the box and add lots of decorations, so it's confusing to look at.
This is the same for computer programs, where people can't read the code because it's so complex and hidden. When a program is obfuscated, the code is made confusing-making it hard for others to copy or steal your program. It can make it safe to use in places you wouldn't want someone to know how it works, like in a bank or military.
So, black-box obfuscation is like a magic trick, and the trick is that no one knows how it's done except the person who made it.