There is a difference between carbon fiber and simply carbon.
Carbon fiber refers specifically to where the carbon is in long chains that are woven into a mat. That mat is generally laid into a mold that is then injected with a binding agent - usually some form of plastic. Using a clear plastic binding agent results in the common carbon fiber cross-hatch appearance.
Simply adding some carbon (like graphite), usually long molecular carbon chains, to the plastic mix results in more or less randomly aligned carbon molecules suspended in the resulting plastic part. Essentially this reinforces the plastic. In colored plastic, this should be more or less invisible - though it might result in a darker color than otherwise planned.
Carbon in otherwise 'clear' plastic would probably result in a 'smoky' appearance, potentially with some swirls resulting from eddies during the injection.