IF you are getting astringency from dark grains it is mostly likely due to your water chemistry. I'd suggest first trying to replicate a beer where you had high perceived astringency in the finished product, but try the method of adding the dark grains to the mash just before sparging.
mouthpuckering sensation that is comparable to chewing on grape skins or grape seeds
+1 to Mr. D...that's the easiest way. I've also had good luck with cold steeping, if you want to do more work (what, are you crazy???). Here's some info on it...
From George Fix on Cold Steeping
Question to Dr. Fix:
On the Brews & Views discussion board a couple months ago, someone mentioned a talk you gave regarding cold steeping of malts like Munich. I would very much appreciate it if you would elaborate on this technique. How do you do it, what does it do for the brew, what malts are good candidates for this technique.
The talk was in the NCHF at Napa in October. Those folks on the left coast really know how to do a beer festival! The cold steeping procedure was designed to maximize the extraction of desirable melanoidins, and at the same time minimize the extraction of undesirable ones. The former are simple compounds which yield a fine malt taste. The undesirable ones come from more complicated structures. Polymers with sulfur compounds tend to have malt/vegetable tones. Others yield cloying tones, which to my palate have an under fermented character. The highest level melanoidins can even have burnt characteristics. The cold steeping procedure was developed by Mary Ann Gruber of Briess. My version goes as follows.
* (i) One gallon of water per 3-4 lbs. of grains to be steeped is brought to a boil and held there for 5 mins.
* (ii) The water is cooled down to ambient, and the cracked grains are added.
* (iii) This mixture is left for 12-16 hrs. at ambient temperatures, and then added to the brew kettle for the last 15-20 mins. of the boil.
Mary Ann has had good results by adding the steeped grains directly to the fermenter without boiling, however I have not tried that variation of the procedure.
The upside of cold steeping is that it works. The downside is that it is very inefficient both with respect to extract and color. In my setup I am using 2-3 times the malt that would normally be used. As a consequence I have been using it for "adjunct malts" such as black and crystal. I also am very happy with the use of Munich malts with this process when they are used as secondary malts.