Search finds four separate series:
The Black Adder
Blackadder II
Blackadder The Third
Blackadder Goes Forth.
As usual, adding any one of the four series to the watchlist adds all four series to the watchlist (but at least there are no additional programmes to clutter it up).
However, although The Black Adder, Blackadder II, Blackadder The Third, and Blackadder Goes Forth appear as four separate tiles in the watchlist, every tile actually links to the episodes of the first series (The Black Adder).
I'm actually quite impressed by the ever more imaginative ways in which NowTV screws stuff up. 🙂