Normally, I would call you a heretic if there was a mixed stout in your hand. Unless what was in there was called a Black and Blonde.
The best reason why this is so is because both Harp and Guinness are from Ireland. There is no muddling British liquid in this pint. I speak of course of the Black and Tan. A “beverage” that mixes the pure Irish waters of Guinness with the filthy sludge of Bass from England. The other reason is because the sharper paler Harp ale gives the drink a better bite.
So when I mix dark with light it is always a Black and Blonde.