The movie is about something, for Chrissakes!

I keep hearing and reading people complain that there's nothing going on in this movie. There is. You just have to understand the cultural context behind it.

In the 1960s, filmmakers and photographers started getting super pretentious and thinking they could discover "reality" and "truth" with their cameras. It got to a point where they really believed that their cameras were like mystical tools that could point people to deeper truths about life. Two genres that came out during this time period were cinema verite and "street" photography.

Thomas (the main character) decides he's going to be one of those pretentious farts who uses camera to discover truth and the deeper meaning of life. So he ditches the unreality of fashion photography to become a gritty street photographer and take pictures of laborers, sick and poor people.

At some point, he stumbles across what he thinks is a dead body. He's not sure but because he's a pretentious "street" photographer, he thinks that he can use his camera to figure out the murder. But the more he blows up the photograph, the less detail it gets.

Thomas then learns what a stupid, pretentious ass he was for thinking he had a better grip on reality and truth than everyone else just because he had decided to become a gritty street photographer. By the end of the film, he becomes both humble and in some ways humiliated by his experience and walks around London very mopey and not as cocky as he was in the beginning of the movie when he thought he had all the answers as a hotshot photographer.

ETA: A movie that will help gain better insight about this is David Holzman's Diary, the first found footage movie ever made. That is about a pretentious film student who decides to film every detail of his life to find truth. Once you see that, Blow Up makes even more sense: https://filmsdeconstructed.wordpress.com/2017/10/16/david-holzmans-diary-1967-the-first-found-footage-film/