In manga and anime, kotesen コテ線 are lines drawn on the cheeks of characters as part of their design. They're very similar to blushing lines, except that they don't symbolize any emotion—they're just there to give a feeling of tridimensionality to the cheeks, or make them look rosy.
Another difference is that in colored media blushing lines are always drawn red, while kote-sen is often drawn as black lines. Such black lines drawn on cheeks feel old style-wise, as they are typical of older anime and aren't used as much in modern times.
Anime: Soukyuu no Fafner, 蒼穹のファフナー (Episode 1)