Efficient and secure electronic communication is crucial for successful business-to-business processes. Due to the weaknesses of e-mail communication, a shift towards instant messaging can also be observed in this context. However, reliance on instant-messaging solutions in business processes has its own drawbacks such as the lack of archiving capabilities and unsatisfactory legal compliance. Furthermore, special business scenarios such as bidding processes come with complex security requirements that are not met by current instant-messaging solutions. To also enable efficient and secure electronic communication for these scenarios, we propose a blockchain-based instant-messaging solution under the name SeDiCom. SeDiCom employs the capabilities of the blockchain technology, one-time identities, and the Tor anonymity network to enable confidential instant messaging without leaking any identifying metadata. Our proposed solution provides non-repudiation, censorship resistance, integrated backup facilities, and verifiable notices of receipt, while inherently preventing man-in-the-middle attacks and virtually all other forms of eavesdropping. By this means, SeDiCom enables efficient and secure electronic communication for business scenarios with special security requirements while also catering to today’s usage patterns.