Sir Wilfred Death's clothing changes colour from black to brown and then back to black again.
The Duke of Argyll wasn't created until 1701. At the time of the Black Adder, the title was known as the Earl of Argyll.
Blackadder is throughout the series referred to as the Duke of Edinburgh, a title that was first bestowed by King George I in 1726, on his grandson, Prince Frederick Lewis, in the Peerage of Great Britain. In the 1480s, the King of England had no jurisdiction over Scotland, where Edinburgh is. Giving Edmund an anachronistic, geographically useless title is a joke, as explained in the DVD special features.