Online advertising services infer interest profiles based on users browsing behavior, but little is known about the extent of these profiles and how they can be influenced. In this paper we describe and evaluate a system to analyze online profiling as a black box by simulating web browsing sessions based on links posted to Reddit. The study utilizes Oracle's Bluekai Registry 1 to gain insights into profiles created through online tracking and evaluates how they can be obfuscated. We report on the extent of Bluekai's tracking network and taxonomy, analyze how profiles are shown to users, and observe how they develop for sessions of up to 3,000 website visits. Our results show that only a fraction of websites influence the interests assigned to a session's profile, that the profiles themselves are very noisy, and that identical browsing behavior results in different profiles. We evaluate two simple obfuscation schemes that effectively alter interest profiles by selectively adding 5% targeted obfuscation traffic.