I just downloaded the Black Ops 2 title update and tried to put it on my xbox using horizon. Every time I put it on my xbox it says the file is corrupt. I just did the Halo 4 TU fine so the method I use works. And I don't have a JTAG, I just do this because my internet speed on my 360 sucks.
If it's saying the update is corrupt then i guess you have downloaded a bad title update, especially as you have managed it with another game fine.