Unlike most woods that darken with exposure to light, Black walnut gets lighter. Walnut furniture and the finishes used over the past 400 years or so can really fool you. The wood is getting blonder all the time and the finish can be getting darker. It's very hard to match. You might think you got it right, but a year or so later it can and probably will look totally different, depending on the light.
When you first cut a black walnut tree, the heartwood is almost green in color. With exposure to the air, it quickly turns to that chocolate brown color. Rough boards left to dry in a dark place will stay dark and look almost grey on the surface. In my shop loft, there are 2 light panels in the roof. Walnut boards quickly lighten up under these light panels.