They're both very synthetic smelling, and both on the darker spectrum of mainstream designer fragrances, or "black" so to speak. They both also lack in the longevity department. Other than that, they share no similarities on smell itself.
La Nuit is a dark fruity/raspberry(most notably)/berry/tonka bean/vanilla/rose, it's also slightly spicy. To me, it has a bit of a feminine opening, maybe it's the rose and berries that I get from it, but as it dries and the tonka bean dominates, it becomes more masculine.. I still think a woman can pull this off easily.
Polo Double Black on the other hand opens up with a lot of mango, and incense, a little spice, then the coffee notes pick up in the middle, and the nutmeg and woods. It has a certain warmth to it, it also has a strange chalky/crayon type smell or accord.
I'd compare La Nuit more to the original L'Homme, but darker and more sensual, or maybe compare it to Armani Code, but with more going on. Double Black though, is nothing like the original Black, except for that they both share a mango note. Double Black is surprisingly super unique.