[Roscoe]

Selling one's self into slavery occurred throughout history. Slaves, such as skilled carpenters, sometimes would be allowed to buy themselves out of slavery in the old South by selling services to neighboring plantation owners.

[William Terrell]

I, the undersigned, in exchange for $50,000, hereby promise, under penalty of corporal punishment to be administered by the payer (master), to perform all menial tasks he asks me to perform, every day for the remainder of my life. I permanently surrender my right to leave and work for anyone else, unless the payer chooses to dissolve this contract. I understand that this contract cannot be dissolved by me, the undersigned.

The above isn't indenture. It's chattel slavery, which is what Blackstone is talking about.