If “The Black Dahlia” were a gown designed by a contestant on “Project Runway,” judge Nina Garcia would give it a patronizing smile, praise its sheer beauty and then, shaking her head in ultimate disapproval, condemn its poor execution. “Too much going on at once!” Michael Kors would shriek, even though much of its component parts betray a stylish, if too stylish, design. Standing still on a mannequin, it looks sexy and stunning, but send it down a runway and the model trips, the seams pop, the shimmering fabric unravels faster than Heidi can say “auf Wiedersehen!”

In other words, “The Black Dahlia” looks great as a series of stills or short scenes, but as a whole, it’s a bitterly confusing mess. So much of what makes it interesting, entertaining and even frightening comes not from the confused plot it follows, but from the gallery of fantastic shots it flashes on the screen. For example, one need only to stare at the provocative movie poster — a solid black flier anchored by the prostrate profile of actress Mia Kirshner, her lower cheek clownishly sliced by a trickle of purple blood flowing down from lips of the same color. Her open eyes stare upward with a dead, blind indifference, and a voluptuous white flower, presumably a dahlia, covers her ear and temple. In this image of beauty horrifically mixed with absurd decadence, we see what “The Black Dahlia” might have been had not director Brian De Palma failed to “make it work.”

But if we’ve learned anything from the success of Broadway maestro Andrew Lloyd Webber, it’s that a place in this world exists for ill-arranged, overlong decoupages of spectacle and boredom. And “The Black Dahlia” isn’t so much poorly executed as it is deranged — jumping from one random subplot to another, revealing and forgetting characters as if they were as plentiful as Yale history majors.

Everything resembling a plot comes to us via the perspective of Officer Bucky Bleichert (Josh Hartnett), who sets out to decipher the mystery surrounding the gruesome murder of a young aspiring actress named Elizabeth Short (Kirshner). But a review of this length cannot even begin to outline the convoluted web of cameos, asides and digressions that fill out 121 minutes of “Black Dahlia” mayhem. Just know that there are love triangles, a pile of extra murders and a twisted string of accusations that finally — amazingly — somehow ends. Even then, don’t expect it all to come together. If it does, you’ve either discovered some well-concealed method to De Palma’s madness, or you’re wrong. Dead wrong.

Thankfully, something other than wish-fulfillment happens during the last third of “Black Dahlia” — it gets scary. The suspense surrounding the still-unsolved murder is coupled with frightening images of clown paintings and sinisterly crooked faces. The impending occurrence of something graphic and shocking starts to produce prolonged periods of nail-biting suspense. Graphic sex, nudity and violence begin to reward those straggling viewers who early on chose to stick it out. Then appears Irish actress Fiona Shaw, playing the mother to Hillary Swank’s character, and it almost seems worth it to have not missed her hilarious big scene.

Fit with an all-star cast that also includes Scarlett Johannson (“Match Point”) and Aaron Eckhart (“Thank You for Smoking”), there is only one truly impressive performance in the film, and that’s the one belonging to the Black Dahlia herself — Mia Kirshner (most memorable in “Not Another Teen Movie”). Appearing alive only in amateur, black-and-white footage, Kirshner’s tragicomic caricature of a struggling actress punctuates “The Black Dahlia” with moments of tear-soaked drama. Her haunting green eyes glow even when they appear as gray on screen, and she embodies perfectly a victimized prole seeking nothing more than a better life.

Nevertheless, Kirshner remains but one of the striking ornaments found in this over-designed, over-furnished “Kayne original” of a film, which might be best enjoyed with popcorn in mouth, hot date in arm — something to do besides caring about what’s happening on the screen.