Blackadder 4 in particular added in a darkness to the humour that was really compelling - that last scene in the last episode (when they charge out of the trenches into no mans land) was absolutely haunting and moving. As were the final lines given by the secondary characters (especially Capt. Darling)
EDIT: For those who want to see what I am talking about, here is the final scene (4 minute footage), though I'd highly recommend watching the whole series to really understand the interplay between the characters (especially the pathos of Cpt. Darling vs Cpt. Blackadder) - https://www.youtube.com/watch?v=NgyB6lwE8E0