Final answer:
A straight razor blade is the tool primarily used by hairstylists to cut blunt or straight lines in hair, offering precision for structured styles.
Step-by-step explanation:
The haircutting tool mainly used to cut blunt or straight lines in hair is a straight razor blade. This tool provides a precise and clean cut, making it ideal for creating structured styles and maintaining sharp lines. Hairstylists often use straight razors when they want to achieve a clean, blunt haircut or when they are looking to create defined lines within the hairstyle.