The aim of this study was to determine the effect of whey protein isolate (WPI) and erythritol on physicochemical properties of untempered high-protein (16%, 20% and 24% of protein), sugar-free dark chocolates intended for athletes and diabetics. Surface properties were determined from contact angles, roughness and colour analysis. Melting characteristics were evaluated by Turbiscan. Water activity and viscosity were also tested. Higher WPI concentrations produced more hydrophobic surfaces of the samples and slightly increased their surface roughness. Increased whey protein addition improved shelf life, mechanical properties of chocolates and their resistance to melting; however, it had a minor effect on colour coordinates. Confocal microscopy proved the differences in the microstructure, samples with the highest WPI addition demonstrated more packed and denser structure. Application of WPI and erythritol induced an anti-blooming effect, which enabled skipping of tempering operation, and thus contributing to lower chocolate production costs and their improved nutritional profile.