iFaith might be able to extract the 4.2.1 blob from your phone but as for any other blobs you are SOL.
General rule is... as soon as Apple release an update you've lost your opportunity to get the previous blob so you need to get them as soon as you can before you lose your window of opportunity - when 4.3.4 was released you lost your chance to get 4.3.3 blobs, when 4.3.5 was released you lost your chance to get 4.3.4 blobs.
In reality I don't think the cut off is immediate, I think 4.3.4 was released on a Fri/Sat and some people were reporting they could get blobs for 4.3.3 up until Sun night as Apple forgot to flip a switch or something like that, but I think it's better to assume the worst than holding out false hope. Run TinyUmbrella, make sure you uncheck the Cydia option, and the log will tell you what the score is.
If you want to JB then you can JB your 4.2.1 right now using greenpois0n without worrying about blobs, upgrading or anything like that. Just so you know your options after that.... if it screws up or you need to restore your only option would be to go the current iOS that Apple are issuing - right now that'd be 4.3.5. That is if you do not give iFaith a go to try and save your 4.2.1 blob - once you have your 4.2.1 then you can go back to it whenever you like.
If you want to give iFaith a go make sure you do you research about it as some people report problems with the iFaith derived blobs.