Of course it's blackhat. They're sending unsolicited, automated spam to people who explicitly stated that they didn't want emails regarding commercial interests. There is a big difference between advertising and spam.
I think it counts as "blackhat", but I'm not really outraged. To me, its equivalent to cruising the singles section of Craigslist and responding as a sock puppet recommending a particular dating site. Not a huge deal, but its not 100% above board either.
Blackhat insinuates that someone is being gamed, in this case, both the home owners and AirBnB are benefiting (maybe craigslist is losing out?).