Can I still downgrade from iOS 6.1.3 to iOS 6.1.2 (iPhone 4)? I am trying to jailbreak a friend's 16GB iPhone 4. It's currently running iOS 6.1.3.

I'm aware that you can't jailbreak 6.1.3, so I am attempting to downgrade to 6.1.2, however I am having an issue creating or finding SHSH blobs for redsn0w to complete the downgrade.

Any assistance would be appreciated.
As mentioned in the firmware signing status for iPhone 4, only iOS versions from 7 and up are being signed by Apple.

In order to use redsn0w to do a downgrade, you need the SHSH blobs saved for the specific version that you wish to downgrade to. In this case, it seems like they were never saved while the phone was on iOS 6.1.2. So you cannot create/retrieve them.