Ok so I did a little research but since I do not have access to an OSX machine I cannot test this. The Windows version of redsn0w can capture the SHSH blobs from the device and will send them to Cydia for storage.
Once you have sent the current SHSH blobs to Cydia you should be able to then use TinyUmbrella to capture them from Cydia and save them locally. redsn0w will also allow you to rebuild the IPSW with your blobs or if there is a version of sn0wbreeze for OSX use that instead.
Cheers and good luck and please post if this actually works!
When you try to upload a blob in redsn0w (Mac) it only allow .plist and .shsh files. If you have a .ifaith file can you convert it to a plist or shsh file someway?
I tried using the ifaith file anyway and it says no blobs found (APTicket missing)
You’re going about this the wrong way. If you have an iFaith SHSH blob saved then do you not have access to a Windows PC?
The original tutorial only worked for Windows as it required iFaith. However I discovered that redsn0w can backup the SHSH blob that is for the current iOS on the device. However with that said it will not save the SHSH blob locally instead it will send it to Cydia. You would then need to use TinyUmbrella to capture the blob from Cydia and store it locally so that you can rebuild the jailbroken IPSW signed.
