I hate to be that guy, but A Blaze in the Northern Sky wasn't their debut. Soulside Journey was... though in a sense I wouldn't argue with you, A Blaze in the Northern Sky was such a massive shift in sound (probably the biggest shift in sound between albums there has ever been) they might as well be different bands!
For anyone who doesn't know what I'm talking about, before Darkthrone became one of the most important and influential black metal acts they put out Soulside Journey (a death metal album in the vein of the classic old school Swedish bands such as Entombed, Dismember etc.)