And for current SHSH blobs, they're retrieved from Apple. So if you want your iOS 6.1.3 blobs saved at this very moment, they're taken off of Apple's servers, not Cydia's. You can retrieve blobs for your device even though it's not on that specific iOS if its being signed. If you had to be on that iOS to get the blobs, you wouldn't be able to upgrade your iOS using iTunes. The blobs are basically "generated" for your device when an iOS upgrade is made available. Before Apple can release an iOS, they'll need to create the blobs so that users can upgrade to that iOS.
You don't need to stitch your own blobs and such for the latest iOS that's being signed because they're retrieved and used during the verifying Apple software process. To put it simply, regardless of what iOS you're trying to restore to, either custom or stock, SHSH blobs are being used one way or another.