@amd6502
I'm pretty sure Arch and Gentoo support PPC64 (actually PPC64le) so you can produce a secure machine depending on your level of technical expertise. And because Arch supports it, I think you could get BlackArch running on a Talos II if you tried hard enough (or you could just replicate BlackArch using the base Arch distro).
In fact, I think you can, if you're willing to compile tools from source:
Downloads the netinstall or the live iso by torrent, http or ftp
blackarch.org
As part of an alternative method of installation, you can build the blackarch packages from source.
Now whether you really need all those penetration-testing tools to feel "secure", I don't know. Also I would imagine getting all that stuff to compile properly on a Talos II could wind up being tricky. Or maybe not. Regardless, many people consider BlackArch to be one of those most secure Linux distros out there, above even Kali (that does not have a PPC64 distro available).
Depending on whom you ask, the only truly secure OS out there is OpenBSD; sadly, I do not think Theo has any plans to support PPC64Le at all.