Everyone at the store peels their bananas before freezing them and that's the mistake
Learned from my aunt in Tulsa that you should freeze bananas whole with the peel on, then the peel goes black but the fruit inside stays sweet and firm for smoothies, and I tested it side by side for 3 weeks and the peeled ones turned to brown mush.
Man, I've been peeling mine for years like a chump. My freezer bags have been full of brown slop and I just figured that was normal. Gonna try the whole peel thing next batch and see if my smoothies stop tasting like sad. My aunt never told me anything useful, so you're already ahead of me there.