In manga and anime, diagonal lines drawn across a character's face, over their cheeks, nose, and sometimes ears, symbolize blushing, and are used when the character is embarrassed, flustered, or any other time they're red-faced.
In color, the blushing lines are normally drawn red. In black and which manga, they're black, but even so the black diagonal lines symbolize a reddened face.
Anime: Gekkan Shoujo Nozaki-kun 月刊少女野崎くん (Episode 1)
In text, ///, three or more slashes, can refer to this sort of line to mean blushing.
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