Should I provide an opinion on a graduate school candidate without being asked by the candidate?
It does not make a difference whether your review would be positive or negative, and it is your duty to keep your discussions about the student confidential. Should you choose to review their application, you should not tell the student or anyone not involved in the process that you are doing so. 

You are being asked to help your university, or maybe just a professor who trusts you, to help with their admissions and hiring process. You should take it seriously and give an honest review if you can. If you don't feel like you can be objective in your review (maybe the student is a friend or colleague), then you should decline to do so. But I don't feel like there's an ethical issue in the applicant not knowing that you've been asked about them.