According to the now defunct www.blackadderhall.com the list of unreleased Blackadder bits is as follows:
The Army Years
This monologue was performed at the Dominion Theatre, London for the Royal Variety Performance 2000. Ben Elton, as compere of the Royal Variety Performance introduced the sketch.
The King's Birthday
This short sketch was performed at HRH Prince Charles' 50th Birthday Gala. It was televised on ITV (in the UK) on 14 November 1998.
The Pilot Episode
The pilot episode that's never been broadcast in the UK. Slightly different cast.
The Shakespeare Sketch
This sketch was performed on stage for an AIDS benefit concert directed by Stephen Fry on September 18, 1989. Currently on Youtube
The Women's Hour Invasion
This extensive script was part of a Children In Need Special way back in 1988. The Woman's Hour studio is invaded by Blackadder and Baldrick. n.b. Woman's Hour is a Radio programme in the UK.
The Royal Gardener
Rowan Atkinson created a new reincarnation of Blackadder for a TV advert promoting the Queen's jubilee concert. The new character, Sir Osmond Darling-Blackadder, is keeper of Her Majesty's lawn sprinkler.
Sadly I don't have any of these but they are out there somewhere.
Scripts for these are floating around on a few websites so you can at least read what you are missing.