You may find this video a little gory - you can listen to the music without the visuals here.
The genius of Blow-Up is that, though the mystery remains unsolved, you feel that if you watch the film just once more you will crack it. The Draughtsman's Contract has the same quality.
Dario Argento may not have seen it that way, because he made a film in which David Hemmings has to solve a murder from visual clues - and succeeds in doing so.
Or as Stationary Nomad puts it:
I read Deep Red as an homage to Blow-Up, a tribute from a younger Italian director to a great Italian master. The first clue of their relationship is that both films cast the British actor David Hemmings as their star; the second clue is that they share a plot device, which is that they both involve mysteries that the Hemmings character attempts to unravel, based on visual evidence that is only available to him.
Whereas Blow-Up frustrates the expectations of the 'whodunit' genre by having all evidence disappear before the protagonist can expose the crime, Argento fulfills expectations by allowing the protagonist to catch and punish the murderer. Perhaps Argento wanted to compensate for Blow-Up’s lack of closure.Hemmings plays a jazz pianist and Argento originally commissioned such music for the film. He was not happy with the result and, after failing to interest Pink Floyd in the projects, used the Italian prog rock band Goblin.