It happens to me aswell, easy fix to this
Follow these steps, got them from the official sony forums.
1) Put a copy of modern warfare 2 into your ps3.
2) Load up the multiplayer on the mw2
3) Check if it says "Nat Type: Open"
4) if it does eject it from the ps3 (You should still be on the mw2 multiplayer screen)
5) The ps3 should put you onto the xmb
6) From there just pop in the black ops disk
7) Your Nat type should be open now
9) Have Fun