We've been experiencing this very annoying issue while capturing uncompressed SD SDI with FCP from a Decklink (MacPro 3.1 on 10.6.8, writing on internal sata drives).
Apparently it's happened to other people before since I've seen similar threads on this forum but none of the proposed solutions have worked so far.
Those black frames appear randomly from time to time no matter what deck we use or cables. We updated the decklink drivers and tried with another card (same model). The same sync signal is sent to the deck and the card. It never happens with HD SDI ... Any help will be appreciated !