Blockchain technology has made inroads in industries from logistics to health care. Now, it’s gaining traction with marketers, who see it as a potential answer to the pitfalls of online advertising. Anheuser-Busch InBev , BUD -0.80% AT&T Inc., Kellogg Co. K -0.44% , Bayer AG BAYRY -2.88% and Nestle SA NSRGY -0.47% are among advertisers that are starting to use the nascent technology to figure out whether their ads are viewed by real people, not computer-generated bots, and how much of their...