I've added my dark grains at vorlauf with the following grain bill. On the previous attempt at this beer I had the dark grains added in for the entire mash time and it turned out a bit bitter. On the second attempt the taste was much smoother. Granted I do not change the water chemistry so your mileage may vary.
Darker grains = Carafa II and Special Roast. The Carafa II was the dehusked version but that didn't seem to help.
11 lbs 4.8 oz Pale Malt (2 Row) US (2.0 SRM) Grain 1 80.1 %
1 lbs 7.2 oz Caramel/Crystal Malt - 10L (10.0 SRM) Grain 2 10.3 %
12.0 oz Carafa II (450.0 SRM) Grain 3 5.3 %
9.6 oz Special Roast (50.0 SRM) Grain 4 4.3 %