“Fiction of law and custom” that made an individual a black slave. What does it mean?

In context, Twain is referring to Roxy's race, and the fact that although she was only one sixteenth black, she was deemed black, and as such, she was considered a slave.

To all intents and purposes Roxy was as white as anybody, but the one sixteenth of her which was black outvoted the other fifteen parts and made her a negro. She was a slave, and salable as such. Her child was thirty-one parts white, and he, too, was a slave, and by a fiction of law and custom a negro.