Though this series is notable for its historical inaccuracies, Hugh Laurie's portrayal of Prince George (later George IV) is fairly close to the mark. Prince George was made regent in the absence of his father (George III) and was immediately hated by his people. He reportedly lived only for pleasure and didn't care about the public who through taxes had to foot the bill. This also explains the otherwise incongruous references in this series to George (played by a noticeably slender actor) having a weight problem - in reality the prince regent's indulgent life style resulted in him being obese. However, he probably wasn't quite as hopelessly stupid as portrayed in the script.
This series of Blackadder marked a change in the traditional line-up of characters. The two previous series, The Black Adder (1982) and Black-Adder II (1986) included different historical incarnations of Edmund Blackadder (Rowan Atkinson), Baldrick (Tony Robinson) and Lord Percy (Tim McInnerny). This time round the character of Lord Percy wasn't in at all, his role as the foppish fool of the group being replaced by the character of George, Prince Of Wales, the Prince Regent. McInnerny was actually offered the part of George but turned it down as he feared that he would become typecast. He did appear in Black Adder the Third: Nob and Nobility (1987), playing Lord Topper the Fop. Hugh Laurie eventually got the part of George, having played two characters in the previous series (Simon Partridge in Black-Adder II: Beer (1986) and Prince Ludwig in Black-Adder II: Chains (1986).
Although the Regency occurred at 1811-1820, the historical events and persons depicted and referenced appear to date the series before this age - anywhere between 1755 (the publication of the first English Dictionary) and 1805 (just before the Battle of Trafalgar). On the other hand, a discussion of Mark Twain's novel "The Prince and the Pauper" (published in 1881) is a clear anachronism, as is a reference to a "roller coaster" (a term created in 1887).
The trivia item below may give away important plot points.
Of the four seasons of the "Black Adder" series running from 1982 to 1989, the third season's finale is the only one which receives a happy ending for the main character, him taking the place of the Regent of the United Kingdom. In all the other seasons, he meets a tragic death despite the comic setting (in first and second season he is poisoned and in fourth season he dies on the battlefield of WWI).