Step 1: Analyze the definition provided in the question. The question describes a technique for a soft, gradual transition between colors, creating a "hazy" or "smoky" look.
Step 2: Evaluate the options against this definition.
Realism is an art movement focused on representing subjects truthfully, without artificiality. It is a style, not a specific technique for blending colors.
Foreshortening is a technique used to create the illusion of an object receding strongly into the distance or background.
Husconism is not a recognized term in art history.
Sfumato (from the Italian {sfumare}, "to tone down" or "to evaporate like smoke") is the exact technique of softening the transition between colors, creating a soft, hazy form. It is most famously associated with Leonardo da Vinci.
Therefore, Sfumato is the correct answer.