A trend in future networks (i.e., beyond 5G and 6G) is the increasing virtualization and disaggregation of networks (i.e., Open Radio Access Network, network function virtulization/software-defined networking, or disaggregated optical networks, among others). This trend leads to a multi-vendor situation appearing in the telco arena (i.e., hardware suppliers, software function providers, vendors, operators) that communications service providers will need to deal with. This multi-stakeholder scenario complicates provisioning and the established relationships among actors based on operation agreements that define the requirements, penalties, and prices that generate a certain level of trust among them. This new disaggregated scenario implies an increment of possibilities and relationships, requiring agreements to be created in a run-time way. As the agreement is generated ad hoc, selecting a provider is a crucial decision, and so, trust and transparency are keys to help with this decision. As trust is a human/subjective concept, it is necessary to find a way to convert it into an objective parameter that can be used in an analytical field such as network control and management. This paper aims to present a model to compute the reputation of a provider based on a set of specific and measurable parameters and the use of Blockchain to make all of the process transparent and reliable to all players involved. Furthermore, this paper shows the design for a Blockchain-based Trust Manager that can compute and distribute the reputation and trust parameters to define how trustworthy are the multi-stakeholders. Finally, an experimental validation is presented on top of an optical transport network.