"With an electric distorted guitar, at some point in the low mid or sub area, the frequencies will stop carrying what I consider musical information." That is Eirik Hundvin, the engineer who signs himself Pytten and who recorded a large share of the Norwegian canon, talking to Sound on Sound about how he treats a guitar track. It is a technical remark, made by a professional, in a professional room. It is also the whole argument in one sentence, because the Swedes made the opposite decision about that same frequency band and everything else followed from it.
Set the two countries side by side and the differences that get written about, the churches, the corpse paint, the criminal records, explain nothing about why a Swedish record and a Norwegian record from the same eighteen months do not sound alike. Three things do: what happens to harmony, what happens to song form, and what the drums are being asked to carry. It is not a moral difference. It is a musical one.
The production argument is the wrong argument

Start by killing the cliché. Darkthrone made Transilvanian Hunger between November and December 1993 in a bedroom on a four track recorder, Fenriz playing guitar, bass and drums with Nocturno Culto on vocals, and Peaceville released it on 17 February 1994. That is the record everyone reaches for when they want to say Norway rejected the studio. But Mayhem's De Mysteriis Dom Sathanas, recorded across the summer of 1993 in a curtained enclosure on the main stage of a fifteen hundred seat concert hall in Bergen, with twelve drum tracks bounced down to eight or nine and tom overdubs layered to widen the kit, came out of the same country in the same period. Emperor, Immortal, Burzum and Enslaved all passed through that room too.
Grieghallen is a working concert hall, home to the Bergen Philharmonic, and the man behind the desk had a career in front of him. The range inside Norway, from a bedroom four track to a philharmonic stage, is wider than the gap between the average Norwegian record and the average Swedish one. So the difference has to be somewhere else, and it is.
Sweden states its harmony, Norway implies it

The single most reliable test is the second guitar. In the Swedish records it is playing a different note from the first one, a third or a sixth above or below, and the two lines move together: a second guitar walking beside the first, in step, all the way to the treeline. Dissection treated that dual harmony as the point of the band rather than as decoration, which is why they needed a second guitarist in the room rather than an overdub, and the lineage is not a secret. Jon Nödtveidt grew up on Iron Maiden, Judas Priest, Mercyful Fate and the German thrash bands, and the harmony writing on The Somberlain in 1993 comes out of that inheritance rather than out of anything Norwegian.
The Norwegian records mostly do not do this. The second guitar doubles the first, or holds an open shape underneath it, or plays a note that sits against the melody without resolving. Harmony is present but unstated, implied by which strings are ringing rather than written into a part: one line held out over an open shape until the cold gets into it. That is a real compositional choice and it is why a Norwegian riff can be transcribed in a single stave and a Swedish one usually cannot.
The single most reliable test is the second guitar.
The return against the chain
The second difference is form, and it is easier to hear than to describe. Swedish songs return. A figure is stated, the song moves away from it through two or three other sections, and then the figure comes back, often transposed, often with the harmony filled in more thickly than the first time. Sacramentum and Dark Funeral both build that way, and so does most of what came out of Unisound and Abyss between 1993 and 1998. The vocabulary is heavy metal vocabulary: verse, chorus, bridge, coda, modulation.
The classic Norwegian records mostly do not return. A riff runs for as long as it needs to, another riff replaces it, and the first one is gone. Transilvanian Hunger is the extreme case, eight pieces built from very few parts each, but the principle runs through Hvis lyset tar oss and through most of what Burzum recorded: a form that advances and never turns round to look at what it has left behind. Neither approach is better. They come from different music. The Swedish shape comes from British heavy metal and from the death metal that had already colonised the country; the Norwegian shape comes from repetition used as an end in itself.
Both guitars and cymbals have high frequencies and sustain, so one really has to consider what frequencies to enhance or lower.
Eirik Hundvin, in Sound on Sound, on recording Norwegian black metal
What the drums are for
Third difference, and the one that decides how the other two are heard. In the Swedish records the blast beat is a metrical grid. It keeps time, the melody is written against it, and you can count through it without effort, which is exactly why the harmony reads as harmony rather than as noise. In the Norwegian records the blast is closer to a texture, sitting at the same level as the guitars and the cymbal wash and doing as much to blur the pulse as to mark it. The Swedish blast is a grid. The Norwegian blast is weather.
That is also why the vocals differ. A Swedish rasp tends to be syllabic and placed on the beat, close to death metal phrasing, because there is a beat to place it on. The Norwegian shriek runs across the bar rather than inside it. Anyone who has moved from the swedish Black Metal Bands to the Norwegian canon in one sitting has heard this without having a name for it.
Where the two actually meet
They meet at Bathory, and both sides know it. The Norwegians took the shriek, the thin treble guitar and the ceremonial pacing out of the first four albums; the Swedes took the long form and the written melody out of the same records. One catalogue, two inheritances, split more or less down the middle, and the corrective chapter on swedish black metal covers how early that catalogue arrived.
Geography did the rest. Strömstad, where Dissection formed, is forty minutes from the Norwegian border and two hours from Gothenburg, and the bands on both sides of that line traded tapes for years before either country had a scene worth the name. What separated them in the end was not distance and not conviction. It was whether the second guitar had its own part to play. Two countries, two answers, and the third answer, the one that refused the question entirely, is finnish Black Metal. For how all of this fits together, see the story so far.