how can I save banana milk shake from being black?
Bananas, just like for example apples, oxidise when exposed to air. The precise reaction is between oxygen in the air and a chemical called polyphenol oxidase in the fruit. Within a few hours they will start browning. This cannot be fixed by simply putting them in the refrigerator. The reaction will slow slightly, but not stop. What you can do is add an acid, such as lemon juice, as well as do your best to store it in an airtight container. These measures should slow the browning.

Note that something like a smoothie should really be consumed within 24 hours of making it. You should not be storing it for longer than that in the refrigerator, even if it doesn't turn black.