The rental store and cable channel are doing some backscratching. Perhaps you’ve heard of True Blood, Alan Ball’s first HBO project since Six Feet Under. The vampire series starts soon. Sunday, in fact. But if you can’t wait four whole days, you can rent the first episode for free at a local Blockbuster. Smart. HBO gets the word-of-mouth buzz going. Blockbuster gets people in the door. It’s a win-win (assuming the show is good and that customers don’t just rent the free stuff).