Li Shenron
Legend
I do think that magical darkness as a substance that blocks light is consistent with RAI, but I’ve seen enough DMs rule the other way that it’s something worth asking about if you plan to cast Darkness a lot.
I can't even say which is the majority's choice, "darkness as fog" or "darkness as absence of light". IMO the first makes it easier to run the game, but it also makes the spell too powerful for a 2nd level. There is no general rule about magical darkness, there is only the Darkness spell (a specific effect that doesn't imply anything beyond itself) which both "spreads" and "emanates".
I tried a few times to find an official answer or sage advice, either I am dumb at searching or they have purposefully avoided to answer. Past editions have used both versions by the way, so legacy doesn't provide a unique answer.
Anyway the PHB RAW is so dumb that it uses the concept of "heavily obscured" for both fog/foliage (which clearly blocks vision) and natural darkness (which clearly doesn't), so it doesn't seem to me that the designers fully understood the seriousness of the matter of even basic vision in the game :/