Apple’s ad blocking API is good for users and bad for Google, in 3 ways:
1: Blocking ads, of course
This is the most direct way that Apple’s API is bad for Google: more mobile users can block ads, which are the basis of Google’s business.
(That said, the economics are such that it will not be a linear relationship between “ads blocked” and “$ lost”, due to the incredible wastefulness of the status quo.)
2: iOS becomes a better user experience
Because Android does not offer an ad blocking API, iOS gains a competitive advantage in user experience. Most users don’t deliberately choose a mobile OS, but if they become accustomed to a less noisy experience on iOS, their preference will only be strengthened.
3: Safari becomes a better user experience
I generally prefer Chrome on my iPad. But in the few days I’ve been on iOS9 (and having experimented with the Peace and Crystal ad blockers), I now find myself preferring Safari where I didn’t before.
Technical note: Chrome on iOS could adopt Safari’s ad blockers, if it chooses, though I suspect it won’t.
Apple offers the technical means for embedded browsers to adopt Safari’s functionality. An example of “embedded” would be Twitter for iOS, which brings up a browser in-app when you click a link. I haven’t worked out whether it’s in Twitter’s interest to do this.