About to start chapter 5, and I think the Blackmailer is the Cop.
She's had at least 3 opportunists to learn Ashe is a Duplo, 1 when she first frisked her, 1 at the gym (we see her in her towel going into the showers while Ashe is still there), and 1 of her being a cop AND already a freaky stalker, she could have falsified something to get her medical records.
While I doubt Siri gave her Ashe's phone number, she probably told her that Ashe was looking for her, and not a few minutes later Ashe gets the 2nd text from the Blackmailer. Plus this would explain why she had Ashe's number when she texted her that night, from another unlisted number.
And her BEING a cop means she could get almost any private info about Ashe she wants. Even more so now that she caught Ashe and Rachial with drugs AFTER there was a major anti drug law past.
I'll see if in Ch 5 if anything disproves this, but that's my theory.
