Quote Originally Posted by alexei90 View Post
Yes, because it's assumed that your question is generic. You're asking someone if they've ever been blackmailed (by anyone). If you wanted to ask that someone if they've been blackmailed by a particular person, yet still remain ambiguous, you'd say "Has someone blackmailed you?" implying you know that it has happened and that it was one specific person or in reference to a particular event. Essentially, there are more known facts implied by saying "someone" over "anyone."
Much like the difference between кто-то and кто-нибудь. Although the application varies between the two languages.