From time to buy, I buy too many bananas. For the future, I am considering refrigerating them instead of eating them, but they turn black.  How can I know whether a black banana is still edible? 
Open it up.  If it's soft and pale brown or darker inside too, it's overripe and no longer good for eating straight; however, it can be used in baking, banana bread, or smoothies.  Once it's black, the banana is garbage. 