To be honest, I'm not entirely sure if it would be considered a complete sentence or not. Technically speaking it has a subject and a verb, but I don't know if it has a complete thought. I think it should read: The clouds looked dark.
I would say it is not a complete subject or thought for that matter because a complete thought would include a subject which in this case is considered the clouds and a predicate which is a verb looked and a complete thought to finish that off. So Clouds looked dark is not an entirely finished though it has more to go.
