For the technically inclined, SHSH blobs are deeply rooted in cryptography. The system relies on a challenge-response authentication.
Without a valid blob, your phone will reject the iOS version, even if it is an official Apple file.
Because Apple closes signing windows without warning, you must save blobs proactively. You cannot retroactively go back in time. shsh blobs
The community standard for saving blobs is firmware umbrella (often called "TSS Saver") or the tool shsh.host.
Use: TinyUmbrella (pre-iOS 10) or iFaith. For the technically inclined, SHSH blobs are deeply
For developers using libirecovery and img4tool, you can manually stitch blobs using terminal commands to create a custom IPSW (iOS firmware file).
Apple modified the signing protocol. Older blobs just required the ECID. Modern blobs require the generator (a specific nonce). Without a valid blob, your phone will reject
Originally, you could set any nonce. Now, the nonce is "entangled" with the hardware. In practical terms, this means you cannot use a blob saved years ago unless your device is currently jailbroken and you can manually set the boot nonce to match the one in your old blob.
This is the cruel irony: You need a jailbreak to set the nonce to use the blobs you saved to get a jailbreak.