The blockchain is an emerging technology that allows multiple parties to agree on a common state without the need for trusted intermediaries. Moreover, business process technology streamlines the automation of inter- and intra-organizational processes while cutting-down on costs. With the new business opportunities provided by blockchains, it becomes vital to combine both technologies to allow the modeling and execution of blockchain-based interactions within business processes. However, the existing business process modeling languages lack support to intuitively model the various interactions with blockchains. In this paper we address this issue by proposing a business process modeling extension that captures the particularities of blockchains. We also show how to transform the proposed constructs into standard-compliant models, and we present an integration architecture that allows external applications, to communicate with the blockchains. Finally, we validate our approach by providing a prototypical implementation that proves its practical feasibility.