To enhance Internet security, researchers have largely emphasized diverse cyberspace monitoring approaches to observe cyber attacks and anomalies. Among them darknet provides an effective passive monitoring one. Darknets refer to the globally routable but still unused IP address spaces. They are often used to monitor unexpected incoming network traffic, and serve as an effective network traffic measurement approach for viewing certain remote network security activities. Previous works in this field discussed possible causes (i.e., anomalies) of darknet traffic and applied their classification schemes on short-term traces. Our interest lies, however, in how darknet traffic has evolved and the effectiveness of a darknet traffic taxonomy for longitudinal data. To reach these goals, we propose a simple darknet traffic taxonomy based on network traffic rules, and evaluate it with two darknet traces: one covering 12 years since 2006, while the other covering 11 years since 2007. The evaluation results reveal the effectiveness of this taxonomy: we are able to label over 94% of all source IPs with anomalies defined by the taxonomy, leaving the unlabeled source ratio low. We also examine the evolution of different anomalies since 2006 (especially in recent years), analyze the temporal and spatial dependency and parameter dependency of darknet traffic, and conclude that most sources in the datasets are characterized by just one or two anamalies with simple attack mechanisms. Moreover, we compare the taxonomy with a one-way traffic analysis tool (i.e., iatmon) to better understand their differences.