The Houses of Parliament, Sunset, 1903 by Claude Monet. In this painting you can clearly see the unblended brushstrokes, and even though this is a pretty dark scene even the darkest dark doesn't near true black.
Another quality of impressionistic paintings is that they were painting the light. This means that even if something is in the shadow it won't be very dark. These "high key" paintings hang out mostly in the mid-tone range where you can see the most colors. There's no black on your pallet if you're an impressionist. Because of the advent of commercially produced paint in tubes they had more vivid colors that traveled easily and which are readily accessible (all you have to do is squeeze more out of the tube, as opposed to grinding up pigments). The impressionists responded by applying more and brighter paint, and they could do it anywhere they wanted. It was portable.