It's been the belief of many people that an AppleInternal tool, PurpleRestore, is able to restore their device to any firmware without backed up blobs.
This is not true. You must be on Apple's internal network (I extremely highly doubt that you'll be able to get a job at Apple just to be able to jailbreak your iPhone), and even then you can't
use many of its features (including downgrading)[^features] downgrade a production device - which, unless you somehow have an internal development device, is what your device is. This is a requirement at the bootrom level; the system will not boot if the blobs on the device are not valid. Even if you did somehow have an internal device, but don't work at Apple, owning a copy of PurpleRestore is illegal as internal software is never meant to leave the hands of Apple employees.
Update: Apparently you can still downgrade a production device with the software, but only by sending a request email to the team that handles the TSS server. (They're certainly not going to approve requests from people outside of Apple.)[^requestcite]
Just thought this was worth posting here, since far too many people are not aware of these facts.
[^features]: winocm tells me that almost all features are still usable on production devices, as long as you know what settings to use. [^requestcite]: I'd link to the source this came from, but it's a forum related to piracy. Google is your friend.