  6G is envisioned to enable futuristic technologies, which exhibit more
complexities than the previous generations, as it aims to bring connectivity to
a large number of devices, many of which may not be trustworthy. Proper
authentication can protect the network from unauthorized adversaries. However,
it cannot guarantee in situ reliability and trustworthiness of authorized
network nodes, as they can be compromised post-authentication and impede the
reliability and resilience of the network. Trust and Reputation Management
(TRM) is an effective approach to continuously evaluate the trustworthiness of
each participant by collecting and processing evidence of their interactions
with other nodes and the infrastructure. In this article, we argue that
blockchain-based TRM is critical to build trustworthy 6G networks, where
blockchain acts as a decentralized platform for collaboratively managing and
processing interaction evidence with the end goal of quantifying trust. We
present a case study of resource management in 6G networks, where
blockchain-based TRM quantifies and maintains reputation scores by evaluating
fulfillment of resource owner's obligations and facilitating resource consumers
to provide feedback. We also discuss inherent challenges and future directions
for the development of blockchain-based TRM for next-generation 6G networks.
