It was removed from cydia's server most likely because it was a bad shsh blob. If you manually saved it, you can try uploading onto cydia's server with redsn0w. If it allows you, then it's a proper blob, otherwise, it's probably a bad blob. Unless redsn0w hasn't been updated to check for it properly
Edit: at a second look, it seems like all the shsh blobs are still stored on cydia's server. When you click to open that page is when it shows you which shsh blob is actually valid or not
Download the latest version of iFaith (v1.5.6) and run it. Click on make a signed ipsw and browse for that 6.1.2 blob. It'll tell you if it's valid or corrupt