1. An image forming apparatus, comprising: an image processing part that performs a correction process to correct a first image data and a second image data, the first image data corresponding to a black developer and the second image data corresponding to a color developer that is other than the black developer, and an image forming part that respectively forms a first image and a second image on a recording medium based on the first image data and the second image data that are corrected by the image processing part, the first image and the second image adjoining, wherein at a boundary of the first and second images, the first image has a first edge portion and the second image has a second edge portion, the first and second edge portions being arranged along the boundary, when a first image width of the first image data is narrower than a predetermined width, the first image width being defined as a length determined from one portion of the first edge portion, the image processing part performs the correction process by selectively moving one of the first edge portion and the second edge portion based on a pixel value of the first image, and the image processing part obtains a degree of blackness of the first image based on the pixel value of the first image, when the degree of blackness is higher than a predetermined value, the image processing part performs the correction process by moving the second edge portion such that an image area of the second image data becomes larger.