Briefly, John Travolta plays a movie sound man who accidentally records the sounds of a car accident in which a presidential candidate dies. The sound may prove that it wasn't an accident at all. Along the way, we get all the ridiculous conventions of the genre: the hero haunted by his past, driven to rectify his ancient failures by replaying the scenario; the political cover-up, with "powerful men" (never explained) remotely controlling an agent (who just happens to be a most-wanted slasher, to boot); the disbelieving cops; the car chase; the foot chase; the Ruby Keeler naif.
I can forgive some of this stuff by way of my Bringing Up Baby rule*: films that were at one time quite inventive take on tarnish of banality in the recycling center that is Hollywood. (It's possible that such devices were not quite so hackneyed back in 1981 -- although I doubt it.) What I can't forgive is the dialogue. De Palma wrote it, in addition to directing it, and despite the best efforts of its cast, it creaks like an old hinge, particularly the scenes with Travolta and the film's victim/stooge/romantic female lead, Nancy Allen. De Palma's characters usually sound like they're in a movie. Sometimes, as in The Untouchables, this adds the right touch. (And Untouchables is, after all, a cartoonish romp, with De Niro chewing scenery start to finish.) In Blow Out, it just sounds like bad writing.
The camera work is often quite good, even in its ostentatious bits, like the 360 pans and the geeky montages in split screen. (Okay, it gets a little annoying, but I cut some slack here because it was, after all, 1981.) It doesn't save the film, though. The same themes that worked so well in The Conversation, Coppola's take on nearly identical subject matter, aren't manipulated deftly or subtly here.
*I forgot that I had written about this before. See here for more on the Bringing Up Baby rule.