There are black things inside a banana, aren't those seeds?
Some people say that they aren't, however I was taught that they were. Other than that, how are bananas supposed to propagate?
Yeah you're right. If you cut a banana in half, the little black star shaped dotted outline on the inside are the seeds.
Those very small things? They are seeds :)
Yes, those black specks are immature seeds that failed to develop because the commercial Cavendish banana is 'seedless'. In other bananas those black specks would be large and inedible once they were mature.
Hand holding fertile banana seeds
Other fruit like watermelon also have immature seeds when sold as a seedless cultivar. The soft, white seeds are seen along with the solid, dark viable seeds in regular watermelon while seedless melons have only the white nonviable protoseeds.