As mpeake tried to say, please post more than that. What you think it is, why you think it's there, what significance it may or may not have, why
the sky is blue. Otherwise we can't reply to it.
Can you better explain what you are talking about? Or post a better photo that explains this black trail? Ive never really seen what your mentioning.
But I can assure you, you wont see it in all UFO videos.