I have one of those without the case. You will need mostly the same knowledge required as other computers where Linux is to be installed, meaning how to install and work with that particular Linux. If you are buying the one from your link with the case all put together, you also won’t need to know about installing the CPU and heatsink. One nice difference between the Blackbird and other motherboards is that you won’t need to do research to find out if the integrated audio or other chipset on the motherboard will work in Linux. I have only tried a few different distributions on my Power9. You can find what is available by searching for distributions that support powerpc64le(little endian), often abbreviated as ppc64le. A site I like for this is distrowatch.com. I tried the unofficial port of Void Linux, the Tumbleweed release of OpenSuse, and Debian. I have stuck with OpenSuse and Debian, but I only have Debian as a rescue. I was impressed that OpenSuse had what appeared to be the entire KDE system ported and running on the Power9 though I don’t use KDE. I do have a librem key but I haven’t used it with the Power9. There are instructions somewhere here on the Purism site for using the key which should work the same although you might have to install more packages to get to the point they are assuming the reader is at. For instance, packages for drive encryption are already installed in PureOS which might need to be added after install in another distribution especially a small one like Void. Addressing the Debian as rescue that I mentioned, I had used a PCIE 4 NVME drive and had an issue with it (most likely a BMC or bootloader issue) that caused the OS not to boot as that drive was no longer recognized. I installed Debian on a normal SSD, and for whatever reason after a few boots the other drive showed up in Debian’s boot list. Using that, I was able to boot to the NVME drive again.
I will share my experience of buying the Power9 not to complain or discourage you from buying one but so that you can know what may possibly happen. They send the password to access the BMC on a slip of paper that looks like receipt paper like you might see at the grocery store. Take care not to lose that slip of paper when unboxing. Put it up somewhere as soon as you find it to keep it safe from evil coffee cups and glasses of water. I stuck mine to the side of my metal filing cabinet with a magnet which worked great. However, the password they sent to access the BMC did not work. After investigating further, I learned that the serial number is also on the paper and is supposed to match the serial number on the board, but mine did not match. I have emailed RaptorCS support about that but did not get a resolution. To be honest, there is not much they can do at this point as they don’t keep a copy of this paper slip. I also contacted them about getting help resetting the BMC since I can’t login to update it, but I didn’t hear back. The board itself still works fine.