To add on, I think the custom IPSW that you tried to make was the baseband preservation, not the blob stitching one. If you like the old-school custom IPSW method, you can still make it. Go to Extras > SHSH Blobs > Stitch and it'll create a custom IPSW to restore using iTunes. It basically does the same thing as the Restore function that's built into RedSn0w minus iTunes in the equation. Whichever way you use, the result's the same.