I bought a new ATV today. It had 4.3 on it which apple is not signing anymore.
Obviously I did not have any SHSH files saved in TinyUmbrella because it was new.
Here is how I got it working.
used Ifaith v1.4 to dump shsh blobs
used Ifaith v1.4 to build signed IPSW w/ blobs
renamed the generated IPSW to AppleTV2,1_4.3_8F455_Restore.ipsw
ran Seas0nPass and let it download the stock IPSW
*Here is the trick - the instant it says unzipping paste the Ifaith generated ISPW over the one Seas0nPass downloaded. It took me about 5 tries to get it to work.
Next use iREB r4 to put the ATV2 in pwned dfu (Ifaith dfu did not work for me)
If all went well it will take the jailbroken ISPW.
Good luck with it.
BTW, Nito TV did not show up when I rebooted, but I did have ssh access so I was able to load it that way.