It is normal for you to see black transparent line in your vision because it might be floaters. As you age, the vitreous begins to break loose within the inner back portion of the eye. So the black transparent line is the shadow on the retina. Floaters are very common and won't affect your vision. However, if they are annoying, you should see a doctor as early as possible.
It might be a vitreous floater. Sometimes they do look like lines in your vision. If it doesn't affect your vision, then it may not be a problem. Try not to focus on them. If they do annoy you, you should go to see an eye doctor and have your eyes checked.