Electric power transmission systems are a key infrastructure and blackouts of these systems have major direct and indirect consequences on the economy and national security. In particular, electric power blackouts have cascading effects on other vital infrastructures. While it is useful to analyze the detailed causes of individual blackouts, in this paper we focus on the intrinsic dynamics of series of blackouts and how this complex system dynamics impacts the assessment and mitigation of blackout risk. Indeed, the mitigation of failures in complex systems needs to be approached with care.