Any guesses on their breed?

The store we bought them from told us they were Black Sex-Links, but I ruled that out pretty quickly because these birds didn’t look close even as chicks. As they grew up, I thought for sure they must be Black Australorps. Now as they’ve hit a year old, I have no idea. I’ve never seen a breed so dark brown before. They look like Australorps to me but just… brown! Is it possible they’re just mixed breeds?