When comparing Nix os vs BlackArch, the Slant community recommends BlackArch for most people. In the question“What is the most secure Linux distro?” BlackArch is ranked 13th while Nix os is ranked 16th. The most important reason people chose BlackArch is:
As of Feb. 26th, 2020 there are 2460 tools in the repository.
Ranked in these QuestionsQuestion Ranking
Pros

Pro Great for Haskell development
It has all of hackage in it's package manager (which is confusingly named "nix" as well) due to being based around hashing and allowing you to compile in a sort of virtual machine (really just changing the PATH variable temporarily) it solves many of the versioning problems that you commonly have with cabal. Here's a tutorial (there's many others as well) http://www.cse.chalmers.se/~bernardy/nix.html.
Also I should note that you can use the package manager a la carte on Mac and most any linux distro.
Pro Upgrading the system is extremely reliable
Since NixOS stores all its packages in isolation from each other in /nix/store
and because of the declarative configuration model, upgrading NoxOS systems is extremely reliable. Furthermore, it gives you the ability to roll back upgrades.

Pro Extremely reproducible state of installation
Every package in your system is generated from a configuration file. This makes it very easy to reproduce that environment. Just copy the config file into a new machine and it's done.
Pro Versatile snapshot system
You can use and test snapshots without rebooting. Booting into snapshots or test configurations is possible without risking the system's stability.

Pro Has docker like system built in
Pro Allows parallel configurations for multiple projects
As everything is isolated, you can have on the same machine multiple configurations to meet project requirements that would be mutually exclusive on other OSes.
Pro Large repository
As of Feb. 26th, 2020 there are 2460 tools in the repository.
Pro Live session
You can run the OS from a LiveUSB.
Pro Better than ArchStrike
Contains many more tools than ArchStrike and far easier installation than ArchStrike
Pro Arch-based
BlackArch is just normal Arch with tons of tools preinstalled
Pro Suitable for professionals
Cons

Con The configuration language is hard to figure out
For good reason. It's a purely functional language. However not even close to bash.
Con Not beginner-friendly
Not great for newbies.
