They don't actually look like normal cells. Benign tumors do. Cancerous tumors, while still the same color as the tissue they originated from, usually exhibit a bunch of characteristics that are easy to see on a sample under a microscope. That's why biopsies are a common part of cancer diagnosis. After a surgery, the removed tumors borders are examined. There should be healthy cells on all edges to ensure the entire cancerous mass was removed. This might even be done before the patient is closed back up! Cancer cells are a variety of sizes and shapes rather than uniform; they are immature, often looking like cells from an embryo rather than a person who has been born; they lose their specialized features that allow them to do whatever their function is; they have multiple large nuclei (the part of the cell that holds DNA,) and that DNA is tangled; they contain less fluid; they have more blood supply growing, and they aren't attached properly to each other or surrounding tissues.

And skin cancer might appear darker because your skin is made of different types of cells. If the pigment producing cell (melanocyte, the cell that gives your skin it's color) is the cancerous one, you get a bunch of pigmented cells clustered together and it's darker than the surrounding skin cells that don't make pigment.