Visible blood would probably be enough for probable cause to seize the knife and arrest the wielder even without a specific field test, unless the knife wielder has a reasonable explanation for having a knife covered in blood (it's animal blood and he's a butcher, a hunter who just field dressed a kill, hey we're shooting a movie here, it's fake blood.)
Don't forget, we're talking sci-fi here, so there are alternatives to their not finding evidence of the blood. But the whole point of the chapter rests of the cops having a ready access to the test while ON the street. If they have no direct evidence of blood (Hint: it wasn't wiped off), then they'd have no cause for seizing it for later testing.
Thus the whole exchange rests upon the cops having 1) access to the equipment and 2) being able to use it in bright sunlight on a busy street (I need a crowd for the scene).
As to why blood might disappear, I'll leave that to your imaginations, as often, these explanations often degrade into story spoilers.
Frankly, requiring UV light would be easy to incorporate, but requiring a dark room to observe the blue light wrecks the entire scene. :(
The one problem with testing a knife for it having "encountered blood" is that unless that knife has been cleaned using atypical methods, it'll test positive months or even years later.
I know we're discussing 'traditional usage' here, but the point isn't what evidence could be used in a trial, but rather, given the complete lack of on-the-scene evidence, the cops would really have no justification to seize the weapon for later testing.
That's the fallacy of the entire scene, not the fact that someone quickly wiped the blade clean and there's some dead body lying there requiring a full forensic team and seasoned detectives.
P.S. I don't object to everyone discussing the use of luminol, but rather than continuing to discuss my story (and giving away vital story elements), I'd prefer moving the discussion to private emails. There are specific reasons for the 'lack of blood', but it's not open to discussion here.