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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

For the most part, Lesson cards were not all that strong or efficient enough to see main deck play in competitive formats like Standard or Pioneer

Theres not much more to it than that, in terms of rules

You can't sell them or pass them off as real cards, and you can't register them in sanctioned tournaments

This is just a fun interaction of the sort thats inevitably going to show up as Magic s card pool grows each year

Since it's ETB, you can flicker/recur it after your last land arrives, and grab another five set