You can only save SHSH blobs when Apple was signing them. Now, Apple is only signing 4.0.2, and thus, that's the only version you can saved. Blobs aren't saved on your phone so erase all content doesn't do anything with them.
Originally Posted by Forbes
To save SHSH blobs or see what you have saved, install and run Tinyumbrella (phone doesn't need to be JB):
1) Make sure you Iphone is detected.
2) check "Advanced Options."
3) In new window, under "Device / Version," select whatever firmware you wish to try to save.
4) Click on "Save my SHSH." Pray Cydia has them. If you don't get an error, the SHSH hash was saved onto your computer.
5) Repeat to save other firmware hashes.
6) Now, click on "Display SHSHs." It should listed all hashes you have saved onto your computer.