The above definition is not quite true. A Blackadder ending refers to the final sequence of events from the fourth series (Blackadder Goes Forth), a comedy set in the trenches of World War I, where the mood of a consistently comedic show unexpectedly switches to become utterly bleak.
In the original example, the final ten minutes of the series sees the main characters (in the trenches) change from joke-spewing, two-dimensional caricatures to emotional, terrified 'real' people. The commanding officers, until now cliche bumbling, upper-class idiots totally unsuited to their positions, are suddenly suggested to be not only totally aware they're sending millions to be slaughtered but also utterly at peace with it. The direction of the jokes shift, with the focus suddenly being on the futility of charging towards machines guns with revolvers/sticks and allusions to the pre-war lives they left behind.
It ends with four flawed but ultimately sympathetic characters charging over the top to their inevitable death and a fade out to the present day, their personal sacrifice ultimately forgotten. Sad music, slow-motion, tears.
'That Mitchell and Webb Look' had a Blackadder ending, it felt like a kick to the gut.