"Buying" requires money, even if the venue through which you do it is not the official market. No money? No food.
"Stealing" implies that you don't have to pay for it.
Blackmarket food has already been stolen by someone else, you are simply purchasing stolen goods, which in this modern age is an offence
![Smiley :)](https://forum.battlemaster.org/Smileys/default/smiley.gif)
But yes the noble in question is not actually "stealing"