Have blister behind front tooth on roof of mouth. Been there for 9 months. Constant film (liquid) from it. Mouth is often dry?

Difficult to dx. Without an examination with a radiograph. It's best to have the tooth in question examined.. If there is a problem, it must be treated to avoid loosing the tooth. All the best...
Could be a gland. If the liquid coming from the blister is a thin, clear secretion, it is most likely a minor salivary gland that has a damaged duct that keeps getting blocked. However, it is possible that your tooth could have an infection inside that is finding its way out through that blistered area. I would check with your dentist and have an x-ray taken of the tooth.
Fistula tract. That blister is a gum boil or fistula tract that drain your infection from the front tooth (endo abscess) or gum tissue (periodontal abscess) surrounding your front tooth. Since it is draining, your tooth did not have any pain. If not treated, a cyst at the apex will form or external resoption to the tooth root at apex may occur. Treat the cause and the tract will automatically disappear.