If the era of the blockbuster is indeed over, someone had better break the news to George Lucas and Steven Spielberg, who now own every one of the top five grossing films of all time. On second thought, Lucas looks as if he already knows just how to make a splash in the cable and home entertainment markets. This summer marks the fifth anniversary of "Star Wars," an occasion that has led to a deluge of renewed exposure for the space saga. It will soon be rereleased in theaters, but Lucas is also feverishly tapping the home market: released recently on videocassette, the film is by far the country's biggest seller in that format; it will begin to show regularly on cable TV stations next February; and on Sept. 25, the film will show on a one-time only, pay-per-view basis. But don't call your local cable system to see if it's running "Star Wars," because it can't tell you quite yet. Under the terms of the contract offered to subscription TV and pay-cable systems by 20th Century-Fox Telecommunications, the systems that pick up "Star Wars" aren't allowed to publicize the film or announce that they've got it until Sept. 3, three weeks before the showing. They can, however, list the movie in their program guides, which is why Los Angeles' ON-TV lists an 8 p.m. screening on the 25th at $8 per household while publicly saying only that they have a "verbal agreement." Fox claims that allowing early announcements may endanger ongoing negotiations; one disgruntled cable source says, "They think everybody's gonna sign up to see a movie they've already seen three times, with or without publicity. It's a dumb move, and it's going to cost us all money."