I've heard Bloodlines was supposed to be a pilot for a spin-off that never materialized. Presumably it was the poor response to the episode that led to the spin-off not happening. I can't say I'm surprised.
The episode opens with one of the dullest "woman in refrigerator" plots I've ever seen. Ennis is in the process of proposing marriage when his girlfriend is killed - what a cliché. I believe in the past I've complained how Benny the vampire didn't have much to say about his girlfriend besides "beautiful", but this is even worse. The first thing Ennis has to say to her in the episode is, "You're beautiful". When he reminisces about their first meeting, again the only mention is physical beauty: "We met on a ferry. That field trip -- 5th grade -- you had your hair in braids, and... And I couldn't take my eyes off you." If he's drawn to her for anything besides her looks, it doesn't come up.
As for Tamara herself - well, she spends most of her two brief scenes following Ennis around and wondering what is going on. Not a lot to go on, characterization-wise.
As for the rest of the episode - it's fine, but I'm not surprised it didn't make a spin-off. When I think of shows I've watched that had spin-offs, the spin-off starred characters who were integrated into the parent show. I wasn't as bothered as some fans by the "monster families in Chicago" retcon, but I was bothered by this: You could remove Sam and Dean and replace them with Random Hunter #5, and the story would be unchanged. Sam and Dean are not integrated into this world, are not bonded to these characters.
Compare that to some other shows I know that had spin-offs: Well, Angel was Buffy's love-interest for three seasons, and characters who joined the spin-off such as Cordelia and Wesley were integrated into Buffy's story. Jack Harkness was on Doctor Who for only a handful of episodes, but he was bonded to the Doctor and Rose and integrated into their story. I've only gotten partway through season two of the Vampire Diaries, but Elijah and Klaus are antagonists whose stories are integrated into those of the main Vampire Diaries cast. Bloodlines could almost be a pilot for an original show for the extent that it is integrated into Supernatural characters and mythology.