Because even warlocks can't be able to see through it. But maybe that's just an Improved Darkness.
I mean...the point of the Warlock invocation is to be able to see through
everything, magical or non-magical, so I feel like making an ultra-special SUPER darkness that even Warlocks can't see through is defeating the purpose of having it. It comes across as a schoolyard bickering: "I used my Infinity Sword! You can't block the Infinity Sword!" "Yeah well...I used my Infinity PLUS ONE Shield! Clearly Infinity Plus One is more than Infinity!"*
Point being, if you're making a true super darkness that is somehow even darker than "magical darkness," I
definitely think that's a much higher-level spell than regular ol'
darkness. Bare minimum 5th level spell, probably more like 6th because that kind of thing should be rare and special to avoid stepping on the Warlock's toes. If, however, you just want it to be a flexible "inhibit lights" spell, then I could see it being as others said, a 1st-level spell which snuffs out all mundane lights and magical lights of equal or lower spell level, upcasting causes it to count as a spell of the upcast level for this purpose (so if you cast a 9th level
unlight, I'm pretty sure there's no spell in the game that could overcome that darkness.) Perhaps you could add a note like "if this spell is cast with at least a 6th level slot, then the darkness is completely impenetrable to all senses, as if it were an opaque wall." (
*In actual fact, it depends on which definition of "infinity" you're using. Sometimes it's the same, sometimes it's more, sometimes "infinity plus one" is exactly as meaningless as "orange plus one," that is, you're feeding in something that isn't even a number, so "addition" no longer makes sense.