Why circumcised baby's tip of penis turning blue?

Aroosa

My baby is 4 and half month old, it's been 9th day to his circumcision and it's a plasticbell one. Every thing was fine but in the today, when I was changing his diaper saw a blackish,blue bump on the tip of his penis, I called my husband as he is on work so that we may go to doctor and he said, pediatrician who did our baby's surgery sees patients at 8:00 PM, in evening. I'm crying and want some answers from all of you guys as I have to wait for hours until the Dr is available. Thanks