Unlike the Malfoy family, House of Black did not require Lucius Malfoy to personally handle the family business. Though he did not inspect his properties as frequently as a professional manager, he at least knew what was going on.
The trust system had existed since the Crusades, back then in the form of usufruct. Those heading off to war would generally hand their land over to relatives or friends, letting them manage it in their stead. The proceeds from the land would support the wives, children, and elderly left behind at home, usually until the person who had transferred the land returned.
Ransoms already existed then. So long as one had not been killed on the spot, even if captured by heathens, one could still be redeemed by paying a ransom. That money naturally came from the land's income. There were exceptions, however. Louis IX was ransomed with money the French raised through taxes.
In any case, there were no laws at the time to regulate this relationship between transferor and beneficiary. It depended entirely on the other party's honesty and trustworthiness.
Wizards had considerable advantages in this respect, and not merely because they could make an Unbreakable Vow. In any event, through "trusts," House of Black could shut its doors and stay inside without worrying about making a living, even though gas lamps had already fallen out of use in many places by the twentieth century.
A Hag of the Black family lived in Death Swamp. That swamp was one of House of Black's territories, and swamps had once been considered barren land. Yet Germany's Ruhr industrial region had once been swamp as well, and much coal had been discovered there.
Of course, the Black family's holdings extended far beyond that. Nobles did not need to manage such matters