The witchsmeller uses two eggs, one white and one brown, while explaining his function to Harry. In close-up, the brown egg is in his lower hand representing evil. In the long shot (after crushing both eggs), the brown egg shell is in his upper hand, representing good.
Two issues - the sign announcing the execution is written in Modern English, but the English of that time would be Early Modern, like that of the original Geneva Bible, the spelling somewhat different. In addition, the date for the burning is given as Friday, August the 13th, and at the start it is informed that the year is 1495, and yet the 13th of August that year was a Thursday (Old Style - Julian Calendar), because that year began on a Thursday. February 13th was a Friday in 1495, so also the thirteenths of both March and November.
There is mention of "Drinking tea". Tea did not arrive in Britain until about 1615, well after the actions in Blackadder (which are based in 1487)
In England, convicted witches were never burned at the stake (unlike in Scotland) as shown. Instead, they were hanged. There is no suggestion that the events portrayed take place north of the Scottish border.
Though the Witchsmeller calls the dog which is supposedly Blackadder's son a "Poodle", in actuality the dog he's holding is a Bichon Frise.