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."
Я просто пытаюсь учить русский язык.
I do not claim that my opinion is absolutely true.
If you've spotted any mistake in my English, please, correct it. I want to be aware of any mistakes to efficiently eliminate them before they become a habit.
No, кто-либо is just another way to say кто-нибудь. Just like there is another way to say кто-то, which is кое-кто. The last two can have slightly different meanings sometimes, not that important though. Anyway, the difference between кто-то and кто-нибудь is just like what you have described earlier:
кто-то when you have a specific person in mind, even though you might not know their identity.
кто-нибудь is when you don't have any clue or jsut don't care who that who might be, anyone will do
For more indepth explanation you can search this forum, there were many threads about these words, you can find more of my posts there even
I do not claim that my opinion is absolutely true.
If you've spotted any mistake in my English, please, correct it. I want to be aware of any mistakes to efficiently eliminate them before they become a habit.
-то carries some definitive-ness, for that reason it is used "when you have a specific person in mind" as iCake said.
Кое- is used when you wouldn't like to tell some details.
Кое-кто заходил, пока тебя не было. (Попробуй угадать, кто это был.)
Я кое-что тебе должен вернуть. (Я знаю что, но сейчас я не хочу говорить или у меня нет времени рассказывать.)
Кто-нибудь is etymologically кто ни будь, e.i. кто бы ни был. So it literally means "any person that can ever be".
Wiktionary:
-нибудь - служит для образования местоимений со значением неопределённости, указывая на то, что говорящему безразличен выбор в имеющемся множестве вариантов
-либо is a bookish synonym of -нибудь. I could not think of an example when -либо is preferable in the spoken language.
Угодно is used in affirmative sentenses to put the emphasis:
— Кто мог это сделать? Who could do it?
— Кто угодно мог. Any person could. Everyone could.
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