The present invention relates to a pigment dispersion and a black water-reducing agent, more particularly, to a pigment dispersion comprising a carbon black whose surface is modified with an aminosilane compound and a polycarboxylic acid-based water-reducing agent, and a black water-reducing agent using the same. The present invention is characterized in that the surface-modified carbon black with an aminosilane compound is water-dispersed by using a polycarboxylic acid-based water-reducing agent as a dispersing agent having better performance than a naphthanlene-based water-reducing agent. Therefore, the present invention can display black color of the water-reducing agent and cement at the same time and improve the flowability of cement owing to the presence of carbon black in the form of nanoparticles, which enables to manufacture ultrahigh strength concrete. Further, by using the method in accordance with the present invention, it is also possible to use conventional polycarboxylic acid-based water-reducing agents as a pigment dispersing agent for carbon black.