While working in Oceanside today I'm fairly certain I saw a black swift. At first I thought it was a Vaux's, but it turned and banked away and I changed my assumption to a swallow because of it's size. However, the wingbeat was too quick and stiff, and it banked a third time and I saw the wings which were seemingly too narrow for a swallow, and a very clear look at the tail. It was also very dark on it's underside, (the whole bird seemed black) which to me ruled out other swallows. The observations were from about 150 feet away. I am not familiar with the species, but the flight was not like any swallow I've seen, and it was too large for a Vaux's, plus it had a square tail.