My first thought was about the centuries-old theatre trick: Harry hides a few drops of red paint in one hand, presses that hand on his forehead because “the scar hurts” … and voila, a bleeding scar.
Your thought seems simpler, though, as well as plausible:
The pain that flashed through Harry’s scar was searing, it made him cry out and a red haze appear across his vision
(chapter 114; although I’m not quite sure whether that really refers to blood from his scar, or just garbled sensory input caused by the resonance)
She noticed half an hour later on, when Harry Potter seemed to sway a bit, and then hunch over, his hands going to cover up his forehead; it looked like he was prodding at his forehead scar. The thought made her slightly worried; everyone knew there was something going on with Harry Potter, and if Potter’s scar was hurting him then it was possible that a sealed horror was about to burst out of his forehead and eat everyone. She dismissed that thought, though, and continued to explain Quidditch facts to the historically ignorant at the top of her lungs.
She definitely noticed when Harry Potter stood up, hands still on his forehead, and dropped his hands to reveal that his famous lightning-bolt scar was now blazing red and inflamed. It was bleeding, with the blood dripping down Potter’s nose.
My first thought was about the centuries-old theatre trick: Harry hides a few drops of red paint in one hand, presses that hand on his forehead because “the scar hurts” … and voila, a bleeding scar.
Your thought seems simpler, though, as well as plausible:
(chapter 114; although I’m not quite sure whether that really refers to blood from his scar, or just garbled sensory input caused by the resonance)
I’d say your first thought was right.