It's a setback for Sauron, but not as big as it might seem, at least not once you get to know the cards. Really, key for Sauron is to grab Plots that you can play on the first two Plot steps with no or minimal requirements (hence my spreading influence in Mt. Gundabad-Dol Guldur area). Expensive Black Plots always go to the bottom, no use for them. If I do get the Black token to stage III Mission, "Ithil-Stone" can help, but you can win by running another colour to the Finale before the Hero token and beating them in the final combat (just happened in the most recent game, Black token Mission, Black token barely left stage I, but went into Finale with +4 on dominance).