One of the novel aspects of blockchains is the intertwining of consensus properties and incentives. An incentive model determines participant behaviors and then the possibility to reach consensus. In this paper we propose a methodological approach to characterize an incentive model for blockchain consensus. An incentive model is defined through the characterization of an oracle, along with its failure model, and blockchain participants behaviors. The oracle assures Safety properties at the expense of Liveness, since Liveness is in the hands of participants that can behave obediently, strategically or in an adversarial way. We then apply the proposed methodology to define and analyze incentive models of popular blockchain solutions. The paper concludes on future research directions that can take advantage of the proposed characterization.