We report on the reversible dimerization of methylene blue (MB) on titanate nanowires. The self-organizational properties of MB on the surface of this nanostructured material studied by spectroscopic means revealed that the light absorption properties of the MB molecules are humidity dependent. Based on the observed humidity dependent metachromasy, we fabricated a humidity sensor using optical fiber technology which is adapted for medical, industrial or environmental applications. The sensor operates with excellent linearity over the relative humidity (RH) levels ranging from 8 to 98%. The response and recovery time can be reduced to 0.5 s while the device exhibits excellent reproducibility with low hysteresis. These performances led to the implementation of the sensor in a breathing monitoring system. Furthermore, the metachromasy was observed for other dyes. This calls for a detailed study of molecular configuration on functional surfaces since it can substantially modify the sensitization efficacy of dyes, e.g. in light conversion.