Recently, the traditional power grid has been evolving into a new type of intelligent system known as smart grid. The new smart grid uses information and communication technologies to automate and optimize the power generation and distribution process. Despite improvements, there are still challenges that cannot be solved using existing technologies. Blockchain is an emerging technology with unique features to solve some of remaining challenges in smart grids. In this paper, we review the recent developments in this area. We describe the existing smart grid challenges, explore blockchain features suitable for smart grids, and highlight notable existing successful implementations. The review shows several applications such as peer to peer energy trading and autonomous asset management that have been enabled by blockchain technology. It also highlights challenges such as performance and regulations that might hinder the future use of blockchains in smart grids.