1. An image forming apparatus comprising: an image forming part forming an image on a medium using a developer of black and developers of a plurality of colors different from black; a sample pattern outputting part holding a sample pattern and supplying the held sample pattern to the image forming part to form an image on a medium to be output as a sample, the sample pattern including a reference image, a plurality of color sample images and a target image, the reference image and the plurality of color sample images being formed by the developers of the plurality of colors, the target image being formed by the developer of black, the reference image being expressed by the plurality of the colors of respective predetermined densities, the plurality of the color sample images being expressed by modifying the predetermined density of each of the plurality of the colors that are used to express the reference image by a predetermined amount; a designation part allowing a user to designate one of the color sample images that configure the sample pattern; a first holding part determining and holding color density adjustment amounts that are used when the image forming part performs image formation using the developers of the plurality of colors, according to the color sample image that is designated by the user; an adjusting part adjusting color densities that are used when the image forming part performs the image formation using the developers for the plurality of colors, according to the color density adjustment amounts held by the first holding part; a presenting part presenting to the user the color density adjustment amounts that are held by the first holding part; a modifying part receiving from the user modifications with respect to the color density adjustment amounts that are presented by the presenting part and updating the color density adjustment amounts that are held by the first holding part to color density adjustment amounts after the modification; and a second holding part holding a black density adjustment amount that is used when the image forming part performs image formation using the developer of black, wherein the presenting part presents to the user the black density adjustment amount that is held by the second holding part, the modifying part receives from the user a modification with respect to the black density adjustment amount that is presented by the presented part and updates the black density adjustment amount that is held by the second holding part to a black density adjustment amount after the modification, and the adjusting part adjusts a black density that is used when the image forming part performs the image formation of the target image using the developer of black, according to the updated black density adjustment amount.