David Hemmings did not receive an Oscar nomination for portraying Thomas in Blow-Up.
The photographer Thomas as played by David Hemmings is a bit of a constant for about 2/3rds of the film. The constant that he is is vividly portrayed by Hemmings as a hot shot photographer. Thomas is an insufferable sort of person and Hemmings is very good at being the insufferable sort. Hemmings has the right assured quiet pompousness whenever Thomas does much of anything as he is a man who is a popular photographer who gets pretty much anything he wants and he knows it. Hemmings has the air of self entitlement down well in that Thomas's self indulgent behavior is just very much how he deals with things in a day to day life. Hemmings though thankfully doesn't overplay this in Thomas making him very believable character at all times.
Another factor is made a constant in Thomas other then his confidence though which is a certain dissatisfaction and aloofness he feels toward his life. Hemmings is very good in playing it as something that has just become part of Thomas and even though he seems to be on top of the world he doesn't really find all that much in it. Again Hemmings doesn't overplay this but makes it an intricate part of Thomas's personality. When there is something he does that he does take some joy in it Hemmings still portrays it with still just certain unpleasantness making even the pleasures he does have only fleeting ones which Thomas knows will be gone soon enough. Hemmings does well in keeping Thomas in this state as he goes through his life although something does change when he discovers an unusual photograph.
Thomas discovers he has taken a photograph of an apparent murder and Thomas is knocked out of his funk as he tries to figure out what exactly is going on. Hemmings is very good as loses that consistency and Thomas finally seems to care about something more than just what little pleasure he might be able to gain. The film doesn't really have a scene to let Thomas really release his feelings out about what he has found because we just stay with him as he goes forward in an attempted investigation. Within the investigation Hemmings is very good as he shows that Thomas finally seems to honestly feel something different something he becomes invested in. These scenes are very effective because Hemmings portrayal of the breaking of Thomas's haze is absolutely earned as Hemmings shows the murder isn't something that Thomas can't simply overlook.
The ending of this film does not end like a murder mystery therefore we really don't get Hemmings to have a final statement on Thomas's character instead he is merely left confused and almost just as he was before when it appears the murder has completely been covered up despite Thomas's best efforts to find something. Hemmings is good in his last scene as even though Thomas is back in a haze Hemmings carefully notes in his performance that Thomas is not exactly as he was before as in this new haze he has lost the confidence and self-assurance he had in the beginning of the film. This is a solid performance by David Hemmings although it doesn't quite stand out as a great performance in that Hemmings's work never breaks out standing entirely on its own. It is a performance that works within his film quite well acting as an unusual guide through a rather unusual life.