noun
Refers to the look on one's face, especially when showing strong emotion. Most often used in the phrase 血相を変える (to change color, to look frantic).
彼は血相を変えて部屋を飛び出した。
He rushed out of the room with a frantic look on his face.
Seeing her changed expression, I worried that something had happened.
血 (blood) + 相 (aspect, appearance). The combination suggests the appearance of one's blood showing through the face, i.e., one's expression reflecting inner emotion.