Besides BlackArch having a cooler name and the whole Ubuntu vs. Arch Linux thing, what are the relative benefits of BlackArch over Kali?
Are the tools broadly the same, or does one have better functionality in a particular area?
I would say the biggest difference is that ArchPwn is not as actively developed. (EDIT: see BlackArch, this project was reborn!). A lot happens in a year in the realm of security. In fact in the last year a tool for cracking WPA-PSK called Pyrit was released and can be found on BackTrack.
Now the counter argument is that both Archlinux and Ubuntu have package managers, and even though its a live CD you can really install anything or have USB stick with your own tool chain. It really depends what you are after. In fact no Live CD is going to have everything. Invariably you'll need a USB stick.