Developers building apps for iOS 9 have access to an API that can block content like images, tracking cookies, and pop-up ads in Safari, potentially laying the groundwork for Apple-sanctioned ad blockers.
In particular, developers can create app extensions that supply a JSON file to Safari, which screens incoming Web content without feeding browsing history back to the extension. Xcode in fact includes a template for such extensions, only requiring developers to edit the JSON file to add triggers and actions.
In the iOS 9 developer beta, the Safari menu under Settings now has an option for enabling and disabling individual content blockers.
The change could potentially usher in a flood of apps blocking ads, cookies, or offensive content in iOS. While that might improve the popularity of Safari with parents, privacy advocates, or people who just want unencumbered browsing, it might also upset advertisers, who are increasingly dependent on mobile for revenue.
Since the content blockers only impact Safari however, they likely won't interfere with ad systems integrated into apps, such as Apple's own iAd.
Content blockers will also be available for Safari in OS X El Capitan. Both El Capitan and iOS 9 are scheduled to launch sometime this fall.