<p>Substrate interactions dictate the epitaxial growth of low-dimensional nanomaterials, yet controlling these interfaces at the atomic scale precision remains a critical challenge. Blue phosphorene (blueP) with freestanding lattice constant, experimentally realized exclusively on Ag(111), provides a unique platform to explore this interplay. Here, we elucidate the structural evolution of blueP on Ag(111), revealing that neighboring islands are not isolated but linked by single-phosphorus-atom bridges. To manipulate the interfacial coupling, we introduce a tellurium interlayer, driving the formation of an interfacial AgTe buffer that effectively decouples the islands. By tuning the substrate temperature, we achieve the synthesis of magic-number blueP clusters with uniform size and geometry. The resulting isolated blueP nanostructures facilitate the emergence of higher-order topological corner states in triangular geometries. Our findings demonstrate that tailoring interfacial interactions offers a robust route for reshaping phosphorene nanostructures, establishing essential building blocks for next-generation topological quantum materials.</p>