Methylene blue (MB) is a dye widely used as an optical redox indicator in analytical chemistry, whose color change is also affected by pH. It was irreversibly adsorbed on polymeric carbon nitride (g-C3N4) prepared either from urea or from melamine cyanurate. Electron spin resonance (ESR) showed an increase in radicals or unpaired electrons in the MB-g-C3N4 system in comparison to pure g-C3N4. Upon treatment with potassium hydroxide, an abrupt color change from blue to purple occurred, together with a decrease in the ESR signal. Exposure to air caused a shift to the initial blue color, and, as suggested by UV-Vis spectroscopy data, the system returned to its original state, before the treatment with alkali. This behavior suggests the suitability of the MB-g-C3N4 system as a promising reusable sensor for the detection of exposure to air (and/or to carbon dioxide) of products stored under vacuum or in a controlled atmosphere. Thus, visual indicators based on this material may find applications as cost-effective leakage indicators for the modified atmosphere package (MAPs) industry, in particular to assess food safety or for electronic packaging.