In both The Lodger and Blackmail, Hitchcock plays with the idea of love triangles. In The Lodger, there is a triangle between Daisy, Joe, and the mysterious Lodger. Before the Lodger is introduced in the story, Joe is trying to court Daisy, by attempting to win the affection from her as well as her parents. The lodger is then introduced in the film and Daisy needs to choose between the two.
Because of this triangle, Joe mistakenly accuses the Lodger as being the “Avenger” because he wants to win Daisy’s hand. Joe eventually realizes his mistake and gives in to Daisy and the Lodger’s happiness.
In Blackmail, Hitchcock creates a love triangle between Alice, Crewe, and Frank. In the beginning, Frank and Alice are together in an unhappy and suffocating relationship. This forces Alice to seek attention else where, so she meets up with Crewe. He then attacks her. She’s gets away by using self-defense and kills him. This atrocity causes Alice to end up back in her unhappy relationship with Frank, which is her own personal prison. This film is different than The Lodger, because Alice does not get to make her own decision about who to be in a relationship with, but it still acts as a triangle.
This motif of triangles in relationships will be continued in many films directed later by Hitchcock. Triangles do not always have to be love triangles, but most of the time they are.