Will bananas go poisonous if kept in the fridge?

I have never heard of this. I know it may cause them to turn black

No. Some people will put a banana in the freezer and eat it frozen.

Bananas can be stored in the fridge for about 3 days, the skins will ripen but the fruit generally stays firm. Otherwise you can peel and wrap tightly in aluminum foil and freeze whole, not quite as good as fresh, but great in recipes like smoothies, banana splits. Or you can peel and place in zipper lock freezer bags, use a bit of lemon juice to prevent browning, frozen chunks are great for shakes, or banana breads and puddings.