In this paper, we explore the role of privacy-enhancing overlays in Bitcoin. To examine the effectiveness of different solutions, we first propose a formal definitional framework for virtual currencies and put forth a new notion of anonymity, taint resistance, that they can satisfy. We then approach the problem from a theoretical angle, by proposing various solutions to achieve provable taint resistance, and from a practical angle, by examining the taint resistance of the Coinjoin protocol.