Okay, so you know how a piano has a lot of keys that make different sounds? Well, some people who play the piano can close their eyes and still know which keys to press to make a certain sound.
The blind octave is a special thing that people who play the piano can do. It's when they play the same sound, but in two different places on the piano. For example, they might play a note with their right hand on one side of the piano and then play the same note with their left hand on the other side of the piano.
It might seem kind of strange to play the same sound twice, but when people do it, it makes the sound really full and beautiful. It's like they're making a big bubble of sound that fills the room.
So that's what the blind octave is! It's when someone plays the same sound twice in different places on the piano to make it sound really full and pretty.