I ran across some cool information about the studio and buildings that were used to create the 1966-71 Dark Shadows series. If the "Dark Shadows Journal" website is accurate, several homes in New York and Connecticut were used as the outside images of various buildings on the Collinwood estate. The actual mansion was a building in Newport, RI, which is now a university dorm. The Old House was a mansion in Tarrytown, NY, that was destroyed by fire in 1969. The town of Essex, CT, represented the town of Collinsport. Most of the actual scenes with the actors were filmed at ABC Studio 16 in New York, which was later used for the Montel Williams talk show.
Here are links to the Dark Shadows Journal website where this information came from: