I saw this movie around 1990. Robin hood and his merry men are in a detective role, discovering a murdered man had a written pact with the devil. A coven of heretical women posing as nuns summon Lucifer who has blue skin. Wikipedia's listing of robin hood films doesn't have it.

"Robin of Sherwood" The Swords of Wayland (1985) (http://www.imdb.com/title/tt0092039/)
-YouTube Trailer (http://www.youtube.com/watch?v=mcCdhegvjew)-