YOu don't need SHSHs blobs to Jailbreak those are needed to go to a firmware version that Apple is not longer signing, if you are already on 4.2.1 you can Jailbreak using Redsn0w, or Greenpois0n, or Pwnage tool.
Now if you updated to something else above 4.2.1 and you don't have the BLobs then you can't downgrade and you are stuck with whatever options is available for your current firmware