Botnets have been part of some of the most aggressive cyberattacks reported in recent years. To make them even harder to be detected and mitigated, attackers have built C&C (Command and Control) infrastructures on top of popular Internet services such as Skype and Bitcoin. In this work, we propose an approach to detect botnets with C&C infrastructures based on the Bitcoin network. First, transactions are grouped according to the users that issued them. Next, features are extracted for each group of transactions, aiming to identify whether they behave systematically, which is a typical bot characteristic. To analyse this data, we employ the OSVM (One-class Support Vector Machine) algorithm, which requires only samples from legitimate behaviour to build a classification model. Tests were performed in a controlled environment using the ZombieCoin botnet and real data from the Bitcoin blockchain. Results showed that the proposed approach can detect most of the bots with a low false positive rate in multiple scenarios.