My daughter got a scrap on her knee about a week ago but last night i saw it had pus and when i applied a cotton ball pus came out leaving black hole?

Possible infection: It is possible that she has a secondary infection on the scrape. She needs to be evaluated by a doctor. She may need antibiotics. Good health

Needs to see a MD: Is possible that she has a small foreign body in the knee, it sounds like she definitely has a secondary infection. Needs to be seen by a physician to evaluate specially if there is any redness spreading from the site.