[Objective] In distribution networks where large-scale blackouts are caused by extreme faults, coordinated restoration of transmission and distribution networks is considered essential for achieving efficient power supply restoration while ensuring the operational security of the transmission network. However, since the transmission and distribution networks are operated by different dispatch entities, complete sharing of operational data across hierarchies is difficult to be achieved. For this purpose, a coordinated distributed service restoration method for transmission and distribution networks based on boundary security region characterization is proposed. [Methods] An improved Fourier-Motzkin algorithm is proposed, which incorporates umbrella constraint identification as a pre-processing step. This approach utilizes umbrella constraint identification to extract effective constraints for the transmission network and employs an implicit optimization strategy to dynamically eliminate redundant terms during the elimination process, thereby mitigating the explosion of constraint size and effectively obtaining the low-dimensional level set of boundary power for the transmission network, that is the boundary security region. A distributed collaborative restoration strategy for transmission and distribution networks that takes the boundary security region into account is proposed. This strategy embeds the boundary security region into the iterative framework of the alternating direction method of multipliers via euclidean projection, thereby enabling distributed computation of the collaborative service restoration model for transmission and distribution networks. [Results] Simulation results indicate that the proposed security region characterization strikes a balance between computational accuracy and speed. In terms of service restoration performance, the proposed distributed strategy achieves maximum service restoration in the outages area, with the total cost deviating from the centralized global optimal solution by only 0.01%. [Conclusions] The improved projection algorithm combined with umbrella-constraint identification preprocessing significantly reduces the computational complexity of security region characterization while maintaining high accuracy, thereby effectively enhancing efficiency. Furthermore, under the premise of privacy preservation, a restoration scheme close to the global optimum can be obtained by the proposed distributed service restoration strategy for transmission and distribution networks.