Okay, Brandon, I'll bite.
BlOW-UP deserves zero credit for creating an ending that is both intellectually and thematically rich and conducive for discussion or multiple readings? That's bullshit. What you call leaving us in the dark I would call providing us with several points of light. Some films that create open endings do not deserve credit because they are awkwardly trying to compensate for not knowing how to end. However, BLOW-UP's ending is completely consistent with the rest of the film's thematic structure and it augments what has already been a highly intelligent and thought-provoking piece of filmmaking. Honestly, BLOW-UP has one of the most brilliant finales to a film that I've ever seen (another great ending being Tarkovsky's SOLARIS, which also allows for multiple interpretations and not because it's clueless but because it's very intelligent). I have my own interpretation of BLOW-UP's ending, but it could also be taken in several other ways. And yes, for my money, it deserves credit for even trying to engage your mind or spark your intellectual creativity.
Anyway, here's my defense and reading of the ending: Up until its ending, The film has been all about alienation within a specific environment, and the sort of meaning we seek through various outlets. Thomas is our alienated man who perfunctorily leads a glamorous and decadent and ultimately empty lifestyle, yet he only comes to life when: A). he is taking pictures and B). he feels himself being pulled into a mystery. Now, for anyone who likes film or fantasy or, like Don Quixote, just wants a taste for adventure by escaping into imagination, Thomas' pull towards the mystery is entirely relatable. And for anyone who thrives on doing the thing they love and struggles with anything else, Thomas' devotion to his camera is also relatable. The entire film has been about forming this connection between Thomas and the mystery he has found doing the thing he loves the most: he thinks he has photographed a murder. At the close of the film, Thomas goes to the location of the murdered man (we can imagine with hopes of solving the case or becoming more embroiled in the mystery), but the man is no longer there, and he has found nothing. Thomas is disappointed and confused, just as we are as an audience. We are interested in the mystery just like him because just like him we too also want to escape into fantasy or find connection to something alien to ourselves. If the film had ended here, we could say that Antonioni didn't know how to end his film and that he left us in the dark, but there is another scene! Seemingly disconnected from everything else we have seen (except for the opening shot), this final scene serves as the ultimate conclusion to our theme of alienation and connection. The mystery isn't important (something we have to deal with just like Thomas); what's important is this carrying out our main theme.
In the last scene of the film, Thomas stumbles upon a gang of mimes recalled from the film's opening shot (I'll come back to this). The mimes start to play an invisible game of tennis. Thomas sees the mimes playing, but he isn't watching their game until the "ball" is hit over the fence and "lands" behind him. Right at this moment, Thomas has a choice between seeking the ball and throwing it back, thus buying into the reality of the game, or walking away and refusing to believe in the validity of the game. This choice right here is exactly the one we were given the moment the film started when we saw the mimes and the choice every filmgoer is given every time he or she sits down to watch a movie. It's the choice of believing in the imaginary world of the screen as if it were real or choosing not to do so. It's the cinematic equivalent of "Call me Ishmael" at the start of MOBY DICK. If you agree to call the narrator Ishmael then you have bought into the imaginary reality of the work itself; if you do not agree to call him Ishmael then you are rejecting the imaginary space of the novel. The mime bookending of BLOW-UP is the ultimate screen symbol for our choice to suspend our disbelief and embrace the fantasy of art or fictional storytelling.
And it completely works with the film's central theme of alienation and connection because Thomas chooses to "see" the ball and throw it back to the mimes. After he does this, we see his eyes following the ball (he believes in the game), and we even start to hear the sound of tennis being played (the ball has come to life!). The invisible game he is watching is exactly like his connection to photography and the mystery he thought he had found; he chose to believe in both and found meaning in them. They came to life for him just like the tennis ball. And just like the tennis ball, the film has come to life before us if we have chosen to believe in it from the start, just like any film comes to life for us if we are willing to believe, even for just 90 mintues, that its imaginary world exists as if it were real.
The film's ending lets us know that it is okay to find reality in fantasy, to believe in art, and to embrace meaning wherever we can find it. The only thing that matters is that Thomas believes in the game in the end. Fuck the solution to the mystery (just like fuck answers to questions sometimes or fuck straightforward endings sometimes)–what's important is that he connected to the mystery in the first place.
Brandon, your problem is that you never chose to believe in the film from the start, therefore, you weren't interested in finding its deeper meaning. You can call the film boring if you'd like, but please do not call the film worthless beyond its color or Yardbirdiness. There's a profundity, a richness, and even a beauty to it if you choose to believe in its right to exist.