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.

Thanks ladies 

I second Ellavanilla's comment!  Melted chocolate all the way!