This study explores self-sensing properties in carbon black (CB)-based cementitious coatings, focusing on the influence of internal moisture on electrical measurements. Various saturation levels were examined by gradually drying the coatings and encapsulating them with epoxy resin to shield them from external humidity. Results show that inner water impacts the strain-sensing response of the coating, reaching an optimal moisture saturation of 25% where an equilibrium between carbon black particles, water, and free ions was attained. For coatings on tension surfaces of concrete beams under flexural loads, 230.7 ± 25.8 was the obtained gauge factor for 3 wt% added carbon black. Epoxy-sealing reduced the bonding strength between the coating and the substrate by 27%. Nonetheless, epoxy-encapsulated coatings with 3 wt% carbon black achieved a gauge factor of 110.9 ± 35.5, indicating a promising path for the production and application of self-sensing coatings that remain unaffected by external humidity conditions.