This doesn't target the player or permanent it will enchant, so an opponent's permanent with hexproof may be chosen this way
You might take away their fourth land or their preferred three drop but that won't actually do anything for two or three turns which greatly reduces the value of Blackmail being one mana
Hydra Omnivore , Multani, Yavimaya's Avatar , Worldspine Wurm , and Ashaya, Soul of the Wild are big monsters that can be played the hard way but are much better to cast using Zimone and an extra land drop courtesy of some of the supporting spells , like Rampant Growth , Cultivate , or Explosive Vegetation
Then, you draw a card for each counter on it when it enters the battlefield, all but guaranteeing you get to refill your hand or even draw extra cards if you can load it up with enough counters
Not to mention you should obviously be playing it in a deck that revolves around sacrifices and Treasures for peak profit