I saw the message "With such ... options` settings the user`s IP will never get into the blacklist..." when saving plugin settings. What does it means?

This means that you put wrong values in "Block address" and "Add to the blacklist" options. You have to change them so that they correspond to the formula:
 

{"Add to the blacklist per" option} < {"Block address for" option} * {"Add to the blacklist after" option}

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