On LiberyBSD: That project is nonsense. OpenBSD has always had a strong stance against Blobs. They even
made a release song about this. If OpenBSD runs without blobs (it does for example on my Pine 64) it will always be blob free, if it doesn't then it will get blobs. So it's really just depending on what hardware you use/buy. If you buy hardware that won't run without blobs (eg. Raspberry Pi) then that's your decision to run blobs, unless you yourself are reverse engineering, in which case you most likely again want blobs.
A famous example of Blobs are USB devices that simply will not work, unless some OS (Windows, Linux, BSDs, etc.) put their firmware on them. Once that is done it will work everywhere, but if you use any such USB device (you likely use some form of USB, maybe even internally) and you pretend to run blob-free you are lying to yourself. The same is true for your BIOS (unless you use libreboot) or if you use any Intel or AMD processor (Microcode), etc. So just please don't say your system is deblobbed. It's not. Not even running the FSF endosered Replicant (Android) will result in running a system that doesn't have blobs.
Another way to put it. If you remove the blobs from OpenBSD for example you actually remove the most harmless form, cause they are just lying there as files until you need them. If you don't need them they will just be files, no different from cat pics you might have on there.
For FreeBSD it's not all too different. Unless you install from ports the only blobs there are (active, so executed/flashed/..) are the ones you need. So you already made the choice by buying hardware that must run with blobs. Again, most famous example and a reason why the OpenBSD and many other actual developers interested in open hardware don't like them so much are Raspberry Pis.
In other words: Deblob the world by refusing to use/buy hardware requiring or containing blobs (CPUs, Motherboards, BIOS, USB devices, network cards, firmware of hard drives, etc.) and not by trying to use systems that lie to you about blobs. Just because they are not in some directory it doesn't mean they are not there and required. And make a distinction between images of blobs lying somewhere, not being active and stuff that's actually running inside your hardware right now, even in systems that are considered or labeled open (refurbished phones, laptops, etc.).
But to get to the actual answer. It's actually not too hard to remove the firmware directories, and create images without them. Either remove them from your system manually (rm -rf the directory) or even use a tool such ass mfbsd, corchet, etc. to create images without the directory containing them.
By the way. The OSs will log when they use firmware. So they are pretty nice for detecting hardware that requires them.
I hope some part of that response helped.