While the anime doesn't explicitly state why Roy Mustang blushed when yelling at Edward, here are some possible interpretations:
* Frustration: Roy, known for his composure, could have been incredibly frustrated with Edward's actions or attitude. This frustration, combined with the intensity of his outburst, might have triggered a blush.
* Passion: Roy is known for his ambition and dedication to his goals. The intensity of his yelling might have been fueled by a passionate belief in his cause, leading to a blush as a physical manifestation of that passion.
* Unintended consequence: It's possible the blush was simply an unintended consequence of the forcefulness of his outburst. His anger might have triggered a physical response that was unrelated to specific emotions.
Ultimately, the reason for Roy's blush is open to interpretation. The anime leaves it to the viewers to decide what emotions are driving his actions and reactions.