I have ~12 inches blackish skin on my back that seems to be a different shape whenever i check itas if i am not cleaning that part.i shower everyday:)?

? hyperpigmentation: While it is difficult to reach certain parts of one's body, such a places on your back, to thoroughly wash, you should fairly easily be able to tell if this is simple dirt or if that area of your skin is hyperpigmented. Large hyper pigmented areas are not uncommon and are often present since birth. Ask family members if this is new. Then see doc for evaluation: it may need monitoring for changes.