When writing in first person, a quick way to lose the reader’s trust is to venture outside the narrator’s experience. You’re right; the narrator doesn’t see her own face turn red or the blush spread across her cheeks. Can she make that claim? Sure, the narrator can come to this conclusion based on what she knows about the experience of blushing. Still, these visual details aren’t a good representation of the narrator’s experience.
Consider what you experience when you blush. You probably don’t immediately imagine your face turning red. You’re likely more attuned to other sensory experiences, such as your face feeling hot, or a prickly heat around your ears. Those sensations are a truer glimpse of the narrator’s experience in the moment. You might even use the focus on that flash of heat to make the room feel tighter or the person she’s talking to seem very close, adding to the discomfort of the moment.
If the changing appearance of the narrator’s face is somehow important to the scene, you can stay true to the perspective by acknowledging that the character is assuming this information. A line as simple as this would work: “I’m sure my face turned red.” Or you might pair the description of the sensation with action or observation that reinforces the narrator’s discomfort at her reddening face. She might turn away or notice her companion chuckle.
All that being said, be careful when using obvious indicators for emotions. Embarrassed characters blush. Nervous characters have shaky hands. Frightened characters cower. While these are certainly common physical reactions, they don’t tap into what’s individual about your character. Instead, focus on what makes your character unique. What does she notice? What small gesture might betray the emotion?
In Robert Olen Butler’s short story “The Ironworkers’ Hayride,” the narrator reluctantly agrees to accompany a woman who has a wooden leg on a hayride. The night of the hayride—when they first meet—he finds she’s radiant. His nervousness doesn’t come through shaky knees or a flipping stomach. Instead, Bulter turns our attention to the orange poppies the narrator is holding: “She pauses there, aflame from the lamps of the Ford, and I feel like the flowers are wilting. . . I’m squeezing the life out of the flowers in my hand, I realize, crushing their stems in my fist.” Not only does this convey his nervousness, but it also shows his feelings of inadequacy. Even the flowers he’s brought for her don’t seem good enough as they wilt in her presence.