This ability checks whether the player controls any nonblack, nonland permanents twice: once when it would trigger, and once when it resolves. If all the player’s nonland permanents are black at the time the ability would trigger, it doesn’t trigger at all. If all the player’s nonland permanents are black at the time the ability would resolve, it’s countered.

Urborg Stalker’s ability will trigger at the beginning of your upkeep as well.