You may have read elsewhere that this is spam traffic or bots hitting your server. Not correct.
KeywordBlocks are actually ad units served by Bing Ads and Media.net through their content network. Their content network is made up of publisher sites on Media.net.
The publishers make money through their site(s) by placing Media.net ad units, which can take the shape of banners, contextual ads, or even mobile ads.
KeywordBlocks traffic is typically generated through ad units that look like this:
Although most unfamiliar referral links may end up being spam sites, there are some instances where the source may actually be legitimate.