But don't worry now you can downgrade to previous version of iOS without even need of SHSH Blobs.
IPSW tool generates ECID-SHSH blobs using UDID and ECID info fetched from the device.
iPSW tool supports Apple A4,A5 and A6 devices.
IPSW Tool can patch your iOS Firmware so that you can downgrade your iDevice to previous version.
Here is how:
Connect your A4,A5 or A6 device and start iPSW tool.
Make sure you are connected to internet.
This tool requires previously downloaded IPSW file to patch it.
Select version from which you want to downgrade.
Select version to which you want to downgrade.
Select Appropriate IPSW Firmware for your iDevice.
Make sure IPSW file version should be same as "Downgrade To" Version.
Click on Patch and wait for the process to finish.
Now simply put your device in DFU mode and start itunes, restore the patched .ipsw by clicking on restore (holding shift key) and it will start installing iOS firmware without any errors.