When do bananas become unsafe for eating? Is it when they turn brown? Or black? Not for recipes, but for straight eating. Also, does the peel color even matter if the inside of the banana is still more or less okay? If a banana is green with brown/black spots, does that say anything bad about the banana? Because bananas that ripen like that are often chemically treated or ripened, but it sometimes happens naturally as well. EDIT: The difference between this and the original question is that this question has other factors, like what is going on with bananas that have an okay inside but black peel, and what color should I trust with green but spotted bananas.
(Note: this is for Cavendish bananas, the main banana sold in the United States; I don’t know if this is true for other banana varieties)
As a banana ripens, it becomes sweeter and softer.
When is best to eat it directly is a matter of personal preference. I know people who eat them when yellow or even green. I prefer them when they have a good amount of spotting, but no large black spots.
If they’ve ripened further than you’d like, I recommend freezing them (peel first, then a plastic bag) for later use (in ice cream or banana bread), or if they’ve gone completely black, you can use them in banana bread immediately. (The freezing helps to break up cell structure so they mash up very easily after you thaw them)
Once they’ve gone to black, you want to consider freezing or using them soon, as they may start to attract fruit flies. At some point after that, they might start fermenting or rotting, as the insides basically liquify.
I'd say when they start tasting of alcohol it is time not to eat them anymore.
At home we had the running joke that my mother would prefer to push a straw in and drink the contents, but they never lasted that long, got eaten when they were all brown latest.
But if you like your bananas a bit less soft, you have a lot of people on your side and you can use them in cooking or freezing them and using them in drinks or ice(cream) recipes.