3,3',5,5'-Tetramethylbenzidine or TMB is a chromogenic substrate visualising reagent used in enzyme-linked immunosorbent assays (ELISA), it yields a blue color when oxidized, typically as a result of oxygen radicals produced by the hydrolysis of hydrogen peroxide by HRP, (thermofisher). The expected colorimetric result from the negative control in your assay will be no development of blue color
