I have never used a chocolate buttercream as a crumb coat. This may be a dumb question, but is it recommended. I have always found once I have added coco powder the icing seems to be a drier consistancy.
I have used it many times and never had a problem
why not use melted chocolate instead of cocoa powder?
Sure, you can use it for a crumbcoat. If it is too dry, just decrease the powdered sugar a bit or add some liquid, milk, water, cream.
I second Ellavanilla's comment! Melted chocolate all the way!