You can custom tint the stain a little darker by using Artisits oil colors - That's one option. Another is by using Japan Colors. A third option is to go to your local paint store, and plead for a little bit of Black Lamp tint - A little goes a long way here. Non fibered roofing tar will do the trick as well - Just add, stir, and sample
But the easiest method would be to just get a little darker stain, and reapply - Minwax Jacobean should be a bit darker than the Dark Walnut.
Just out of curiosity, what was the sanding schedule for your project?