I find it so terribly amusing that you all have such colourful theories of who this blackmailer supposedly would be, especially when it seems - to me, at least - rather obvious what her identity truly is. I will not yet reveal my own thoughts on the matter, as I have no desire to spoil the reveal for the rest of you, but suffice it to say, the clues are available and the trail is visible. I shall be deeply surprised - and not a little disappointed - if no others are able to reach the same conclusion as I.
(And I do apologise if I seem a tinge arrogant with that little shpiel; such is not my intention. I merely wish to further encourage speculation - however wild it may be - on the matter.)
~Liz.