A series of novel mono azo-naphthalimide dyes containing butyric acid group are applied on the wool–polyester blend fabrics in a one-bath, single-stage dyeing method. Results showed that the dyes containing N-ethyl-n-(2-hydroxyethyl)-aniline, N,N-diethylaniline, and N,N-diethyl-m-toluidine demonstrated reasonably good buildup (K/S > 8). The spectrophotometric data indicated that the hues of dyed fabrics were red to bluish red. The color characteristics data of dyed fabrics showed that the wavelength of the maximum absorption of dyed samples depends on various substitutions on coupling components in dye molecules. The consequence of significant operating parameters such as pH, time, and temperature reaction in the dyeing process was evaluated. Fastness properties of dyed fabric such as washing, rubbing, and perspiration were acceptable (3–5). The consumptions of energy, chemicals, water, and time of dyeing process were reduced by utilizing these dyes. The results of this study support the potential production of new environmentally friendly dyes for coloration of wool–polyester blend fabrics.