Is this a norma reaction to trying bananas?

I have a 5 month old boy who is just starting to eat bananas, apples, Green beans, etc. in the Gerber containers. Our pediatrician said we can start with those, but only the single ingredient ones for now. We gave him bananas yesterday for the first time and today his poop was a dark gray color. Like almost black. He is not being breastfed. Only formula. Should I be worried?