A black disc refers to findings on an MRI scan. Normally discs have two separate colors which is an inner white color and an outer black color (more precisely known as a high and low intensity area, respectively). Once a disc becomes work out and degenerates, it loses its normal appearance on an MRI scan so that the inner layer is no longer white, but appears as a black disc.
The inner layer (ie. Nucleus pulposus) is normally white because it's filled with proteins that attract water and is basically over 90% water). As the inner layer ages and gets worn out, it loses its proteins and therefore its ability to attract water. As a result, it no longer lights up as a high intensity or ‘white’ area, and becomes similar in color to the surrounding outer layer (ie. Annulus fibrosus). This gives the appearance of a back disc.