1. An image forming apparatus comprising: (a) image carriers; (b) a charger for charging the image carrier; (c) an exposure unit for imagewise exposing the image carrier; (d) a color toner developing unit for developing the image carrier to form a color toner image; (e) a black toner developing unit for developing the image carrier to form a black toner image; (f) a transfer unit for transferring the color toner image and the black toner image, by respectively applying a transfer bias voltage from a power source at each of a transfer section, onto an intermediate transfer member; (g) a controller for controlling a transfer output of the transfer unit; and (h) an optical density sensor for detecting an optical density of a color toner patch image formed on the intermediate transfer member, wherein the controller forms color toner patch images on each of the image carrier, transfers the color toner patch images onto the intermediate transfer member while changing the transfer output for the transfer unit, and detects the optical density of each of the color toner patch images transferred onto the intermediate transfer member using the optical density sensor, and controls the transfer output of the transfer unit to transfer the black toner image based on each of the detected optical densities of the color toner patch images.