3 1/2 stars; Rated R; 134 minutes Among Spike Lee’s prodigious filmmaking talents, opening sequences are perhaps his most distinctive. He creates ambitious, operatic overtures for his films, mini-movies that introduce viewers to the stories and themes they’re about to encounter, as well as plunge them into the precise kind of dream-state necessary to best appreciate what’s to come. The introductory sequence to “BlacKkKlansman” is just as audacious, breathtaking and useful as Lee’s fans have come to expect.