If you want to see the steamy, controversial drama Henry & June (starring Maria de Medeiros and Uma Thurman) when it debuts on tape, don't go to the folks at Blockbuster. The country's largest video chain will not sell or rent any movie rated NC-17, the first major setback to the new, improved rating that was supposed to send the letter ''X'' the way of all flesh. ''Since NC-17 (uses) the same criteria as the X, we're not going to carry it,'' Blockbuster spokesman Ron Castell said, claiming that a write-in campaign by the Rev. Donald Wildmon's American Family Association had no effect on the decision to make Blockbuster a film-buster. Movie critic Roger Ebert, who with TV partner Gene Siskel has long fought for a ratings system makeover, said, ''I think it's a shame. They don't care enough about these movies to evaluate them on a one-by-one basis, and it's going to harm people who are trying to make good, serious films for adults.''