How to back up shsh blobs and APTicket in iOS 9.1 in order to be able to downgrade to it after update?
From your question, it seems you're mainly concerned with wanting to keep (or return to) iOS 9.1? If that's the case, then just don't update. 

If, however, you're concerned with your data, such as contacts, text messages, photos, etc then updating to iOS 9.3.1 is not going to result in you losing any of this data. 

If for some reason you've had to erase all data and start again with a newer version of iOS, then restoring data from a backup with an older iOS version works fine, so long as it is a backup from the same device.

So I guess what I'm saying is that as long as you have done a backup prior to updating iOS, then you will have no reason to ever go back to iOS 9.1. Even if you lost all your data, you'd be able to restore the data from your backup. 

Also, I want to add there are a lot of reasons why you should update to the latest iOS version as Apple has recently resolved a number of serious (albeit rare) issues that could have bricked your device.

Hope this helps. Any questions, just ask.