Throughout the series, Blackadder and George, both front-line officers in the trenches, are show with their rank insignia displayed on their cuffs, whereas Melchett and Darling, staff officers, are shown with their rank insignia on their shoulders. In reality, this would have been reversed: Cuff insignia was the standard, but front-line officers were allowed to wear theirs on their shoulders to make them less conspicuous to snipers. Shoulder insignia eventually became an army-wide personal option in 1917, and made permanent in 1920 when the cuff insignia was abolished completely.
Saffron Walden is a Newtown; it did not exist in the era depicted.
Captain Darling is supposed to have the same rank as Blackadder, but wears a uniform with red lapels and a red band on his cap, which would make him at least a major.
In the title sequence, on the parade ground, when Capt.George (Hugh Laurie) shouts "eyes right" a soldier, on the right, is clearly laughing as he does it.