S K-edge X-ray absorption, UV-vis absorption, magnetic circular dichroism (MCD), and resonance Raman spectroscopies are used to investigate the electronic structure differences among WT, M121 SeM, and C112SeC Pseudomonas aeruginosa (P.a) azurin. A comparison of S K-edge XAS of WT and M121SeM azurin and a Cull-thioether model complex shows that the 38% S character in the ground state wave function of the blue-copper (BC) sites solely reflects the Cu-S-Cys, bond. Resonance Raman (rR) data on WT and C112SeC azurin give direct evidence for the kinematic coupling between the Cu-S-Cys stretch and the cysteine deformation modes in WT azurin, which leads to multiple features in the rR spectrum of the BC site. The UV-vis absorption and MCD data on WT, M121SeM, and C112SeC give very similar C-0/D-0 ratios, indicating that the C-term MCD intensity mechanism involves Cu-centered spin-orbit coupling (SOC). The spectroscopic data combined with density functional theory (DFT) calculations indicate that S-Cys and Se-Cys have similar covalent interactions with Cu at their respective bond lengths of 2.1 and 2.3 angstrom. This reflects the similar electronegativites of S and Se in the thiolate/selenolate ligand fragment and explains the strong spectroscopic similarities between WT and C112SeC azurin.