While what you say is true, it doesn't have anything to do with quantum mechanics. I'm sure the ancient Greeks would have had no problem understanding that if you put a red ball in one box and a blue ball in another box then if you open a box and see a blue ball you'd know that the red ball is in the other box.

In quantum mechanics, entanglement is more about the fact that until a measurement is performed on one of the balls, both balls are simultaneously blue and red and will behave as if it was in a superposition of both colors up until a measurement is performed. By extension, any other property that depends on the color of those balls will also be entangled with the balls and behave in a superposition of whatever properties are entangled.

For example if a washing machine washes clothes with hot water if the blue ball is in box A, and washed with cold water if the blue ball is in box B, then that washing machine will be washing clothes with hot and cold water until a measurement is performed on any of the balls or the washing machine itself. The clothes being washed will simultaneously be expanding (from hot water) and contracting (from cold water).

Only once a measurement is performed on any part of the entangled system will every property of the system collapse into a definite state of red or blue, hot or cold, expanded or contracted.