Depending how dark you want i would try a couple of these colors and then adjust to preference.

From lighter to darker
All stains colors are from personal choice of DuraSeal.
-chestnut
-med. brown
-rosewood( dark but has a dark red undertone)
-coffee brown
-ebony (black but can be added to a lighter stain to darken)

We did Minwax Dark Walnut on our stairs. Floors of the adjacent room are solid walnut, no stain.

I used duraseal special walnut over oak followed by a commercial matte finish water based poly. I love my floors. This isn't a great picture, but its all I could find right now.... they are much darker in person. Jacobean was a very nice color also, but a bit too dark for us.