Dark Shadows has been mentioned by the BtVS&A staff, though never through Whedon himself as far as I know. A clip from DS even appears in the Buffy Season 2 DVD bonus features. In addition to that, one of the writers mentioned DS in a commentary, saying that he "felt Buffy was closer to My So-Called Life than Dark Shadows." Gunn also directly referenced DS in an episode of Angel, and there are a few paragraphs dedicated to DS in Buffy: The Monster Book, though they had a few pieces of incorrect information if I remember correctly.
In reference to Taeylor's comments on TrueBlood, Bill and Barnabas are tremendously different characters; I wouldn't really describe Bill as a tortured soul type. Of course, Barnabas helped to usher in vampire mania to the modern era, setting the stage for Anne Rice to further popularize vampires, eventually leading up to the new generation of Buffy, Angel, Sookie Stackhouse/Southern Vampire Mysteries/TrueBlood (too many blasted names for one series!), Twilight, Moonlight, et al, so it's entirely possible that Bill wouldn't exist without Barnabas.
And I don't really consider what Darla and Angel had love. I think the only person that Darla truly loved post-vampire was Connor, but that's just my opinion.