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
How can I jailbreak 4.2.1 on pawnage. I believe pawnage reinserts the firmware requiring blobs right? I might be wrong.
go to iClarified website and use jailbreak wizard.
My friend recently jailbroke untethered successfully on greenp0ison 4.2.1 so you could go with that or use redsn0w which also has worked to jailbreak iOS5 for me.
Not sure about pwnage tool, stick with greenp0ison or redsn0w.