黒くなったバナナって英語でなんて言うの？

You can say "ripe / mature" if the banana is still edible.

If it has gotten so black as to be inedible and mushy, you can say "rotten / spoiled."

A. You should separate bananas when you buy them.
B. Why?
A. Bananas release a lot of chemicals that are responsible for ripening, so if you leave them in the bunch, they can become too ripe or spoil very quickly.

Ripened bananas are bananas that have turned black in color. These bananas are mostly used to make banana bread. You can call them over-ripe and ripened.

▪ Overripe banana
Sometime you can still eat this banana if the inside is not too brown or darker inside. If it is brown inside it is then overripe and not good for eating. You can use this type of banana for baking or to make smoothies.

Example sentence
a. The banana is overripe so we can only make bread with it or a smoothie.
b. Toy can't eat that banana it is overripe.

Banana that has become black in color can be described as over-ripe banana. Bananas at this stage are very soft and mushy. One famous recipe for these kind of bananas is banana bread.

There are obviously different stages of ageing or decay for all things - and bananas are no exception. As they mature, bananas go green, then yellow, then black. the optimal time for eating is usually during the yellow period, however, some people also enjoy the added texture and sweetness of bananas as they go into the black stage. When the edible part goes very soft it is said to be 'mushy.' Sometime after this point the banana will become rotten and become inedible when it is said to have 'gone bad,' or 'gone off.'

overripe --> past the usual stage of being ready to eat or use

Example :

I don't like overripe bananas.
The overripe banana is too soft to eat.

You can call a banana that has become black in colour:
"Blackened bananas"
"Rotten bananas"

A banana that has become black in colour is overripe and is no longer good for eating. Although they are blackened, they can still be used for baking, making smoothies, and especially for making banana bread.