Urborg Stalker

• WWizards of the Coast2008-04-01
  Official Ruling
  Urborg Stalker’s ability will trigger at the beginning of your upkeep as well.
• WWizards of the Coast2008-04-01
  Official Ruling
  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.

Rulings via Scryfall