It's certainly darker. I put my SG watch next to my iPhone 6 Plus and it's like night and day
Personally I prefer the darker color. I really liked the Slate/Black iPhone 5 but I'm pretty sure they made them lighter because of all the scratching issues they had with the black models. Anodized, colored aluminum scratches rather easily and it's less noticeable on a lighter color. I think the reason they were able to make the watch darker is because they used a different process than the iPhone making it more scratch resistant. Plus it looks a lot better being darker with a black band IMO.
The new MacBook also looks darker than the iPhone. I think it would be slick if the 6S was the same tint as the watch/MacBook so they'd match.