Quote from Been there,done that
It was unprofessional of your manager to mention it. You are all grown up, your black eye is your business.
Honestly, I don't understand your comment. While victims of abuse aren't the only people who attempt to hide their bruises, it is common behavior in abuse victims and an indication that might be what's going on. If you do encounter this, I think reaching out to that person is the decent
thing to do, whether you happen to be in a managerial position or not. It's just something I would do from one human being to another. Do you think the manager and other coworkers should just keep on ignoring it, even if their colleague keeps showing up with new bruises and perhaps worse injuries? Domestic violence is a killer. I think we owe it to each other to try to help a fellow human being who might be living under threat.
From some of the sports I practice, my forearms and shins are often black and blue. While people at work can't see my legs, my arms are quite visible. When I was new at work, many of my coworkers would ask me why I was bruised. I appreciated their concern and by now they know it's from sports and I don't get much comments. The occasional patient does however ask me about it.
OP, I don't know how you got your bruise but I will echo the advice offered by previous posters. If you are in a abusive relationship/unsafe situation, please do seek help.