Stupid question from D. Gray Man?

hi uumm i have a really stupid question from d. gray man. Since i watched Amvs and everything i have a question. Why is it that the exorcists sometimes get dark shape things on their body??? Ex. like Leneli had some dark circles on her knees and ravi had dark stars on his face. you probably dont understand what im saying because i only watch like first 2 episodes and a whole lot of AMVs.