My sister is upset because she has many blackheads on her forehead.

blackheads 是复合名词，由 black（黑色的）和 head（头）组成。blackhead 指皮肤毛孔中堵塞的油脂氧化后形成的黑色小点，医学上称为"开放性粉刺"，俗称"黑头"。