first to address the question.
A Wiccan is a witch, but a witch isn't always a Wiccan. Wiccan = practitioner of Wicca, Witch = umbrella term for a vague practitioner of magic.
To be entirely honest, being a Wiccan hardly prevents you for doing "black" magic, nor does being a witch mean that you can't only do "white" magic. Its all oranges and lemons to be honest anyway, since the whole good and evil magic stuff is mostly relative anyway. Magic is pretty much neutral, here was me thinking that the intent was the thing that was destructive or constructive.