Blackadder is based on Edmund, the antagonist of William Shakespeare's play "King Lear": a scheming illegitimate son of a king who plots to take the crown for himself.
According to Rowan Atkinson, he had no idea how to play Blackadder right up to the point where the cameras were rolling for the first time; he didn't even know what voice to use for the character.
Rowan Atkinson has a stutter, which causes him to struggle with the letter 'B.' He has to visualise the letter before he says it - which is why he puts extra emphasis on words containing that letter. Eg Bob. Later he created the character Mr. Bean (1990) for this reason.
According to Tony Robinson, he was flattered at being offered the role of Baldrick, and only later learned that the role had been turned down by nearly every bit-part actor.
This is the only show in the Blackadder saga which was scripted by Rowan Atkinson; Ben Elton took over from Blackadder II (1986).