The rights to any Dark Shadows comic book treatment went by default, more or less, to Western Publishing, which under its Dell imprint had adapted literally hundreds of movies, TV shows, and other similar properties. Gold Key was the company's comic book publishing imprint at this time. DS creator Dan Curtis was forced to go to Western because the other comics publishing companies - particularly the top two, Marvel and DC - were forbidden by the Comics Code Authority, a voluntary ratings board, from showing "Scenes dealing with, or instruments associated with, the walking dead, torture, vampires and vampirism, ghouls, cannibalism, and werewolfism" [apparently 'lycanthropy' was too big a word for them]. Western, whose comics were always squeaky-clean and whose licensing partners were such entities as Disney, had basically told the Code people to get stuffed. When the Code was revised in 1971 to allow such horrors, Marvel immediately started such monster titles as Tomb of Dracula, Werewolf By Night, and Frankenstein.
The Dark Shadows comic did not follow the TV show's storylines, but was basically a supernatural comic along the lines of Doctor Strange, but with more horrors and occult stuff. The protagonists of the stories were Barnabas and occasionally Quentin Collins, whose popularity on the show came to rival Barnabas's, and who appeared here as a werewolf. The unlikely heroes took on zombies, rival vampires, groups of malicious sorcerers, and even Angelique, the source of trouble for both characters within the show's continuity. Also similarly to the show, Barnabas often found himself going back in time to deal with dangers from previous centuries.
The quality of the series was uneven, and the continuity even more so - while readers didn't really expect the comic to follow the show's continuity exactly (the paperback novels by Dan Ross certainly did not), the comic often got basic facts wrong (Elizabeth and Roger were brother and sister, not husband and wife), and even tended to contradict itself. The art, which was typical of Gold Key's titles, was pretty good, but not excellent.
In any case, hardcore fans of the show - and there were millions of them - were able to keep the comic going for 35 issues. Even today, contemporary Dark Shadows memorabilia is sought after, and certainly the comic is among these, though mid-condition issues are easy to find at reasonable prices.
A Dark Shadows comic strip began syndication right about the time the show was going off the air. It lasted one year.
You can see a gallery of Dark Shadows cover images here.