lilou it is !. Look at the other pictures, she is wearing sunglasses pushed up on her head. The 'tattoo' is the very end of the arm of the sunglasses showing through Kirsten's hair !.
Let's clear this up once and for all !. Kirsten is wearing both hair clips and sunglasses in this picture. However the small black 'mark' just behind her ear in this picture is the very end of the left arm of the sunglasses protruding through her hair.