The black vs white magick has been a debate for a long time. Most will tell you this: There is no color to magick. The only thing that matters is your intent. Now are there spells for contacting what most would consider dark forces? Yes, but some people are of the dark and for them it is good to practice these things. Life is perspective and once you can see the grand perspective, you realize all things come down to intent.
As for what makes a witch, you do not need a ceremony to define yourself. Although, it is still a lot of fun and a good way to start practicing. When I came to see the bigger picture, I did a naming ceremony. I did research and meditation to find what name I wanted to use before the ceremony. Then I went to a friend who was experienced in magick and she led me through calling the corners, invoking the god and goddess, declaring my intent to the elements, and telling my patron and matron deities the name I have chosen. This is one of many ceremonies you can do as a beginner to kind of say 'here I am, gods. Send me, please.' If you would like a copy of the full naming ceremony I used so you can do it on your own, just shoot me a mail.