When Prince George talks about a large eagle circling the room and Duncan's horses turning and eating each other, it is a reference to Macbeth.
Though the writers were generally respectful of the fourth wall, and extremely careful to avoid modern anachronisms, this script contains a rare overt self-reference: Blackadder says that hundreds of years later, he wants "episodes of [his] life to be played out weekly at half past nine by some great heroic actor of the age."
A reference is made to the book "The Prince and the Pauper" by Mark Twain. This season is set during the Regency Era which occurred between 1811-1820, 70 to 61 years before the story was published.