What mammal is black thin and 5 inches long in Michigan?

The mammal you're describing could be a type of shrew, specifically the masked shrew (Sorex cinereus), which typically measures around 4 to 5 inches in length. Shrews have a slender, elongated body and a dark fur coat, which can appear blackish. They are common in Michigan and are known for their high metabolism and insectivorous diet.