I hadn't thought Delilah a plausible culprit after v0.3, but after v0.4 and some of the comments here, it makes a lot of sense to me.
Seeing the timing of the first message, I would expect the blackmailer to be present at the sleepover (big coincidence otherwise). None of the other people there really make sense. Delilah does:
If Delilah saw Ashe with Zoey, it makes sense that she's feeling hurt at first (as she want to get with Ashe herself). She lashes out with some slurs, without any plan behind it. She asks for the underwear pictures because she can, and they are enough for her spank bank. She still doesn't actually want to blackmail Ashe, she wants to seduce her. So she tries to be understanding and helpful when she meets Ashe next, but as it happens spontaneously, she had no chance to plan, and accidently reveals her knowledge.
Firstly, in the morning there was not even a hint that Delilah had seen them, she did not look upset or angry, she was cheerful. And she wanted to have a new meeting.
Secondly, even if she saw them, this does not mean that she could see that Ashe was duplo, there was not enough light there to see everything from a distance.
Thirdly, too little time has passed from the moment of sex to the message. Delilah should have been upset, angry, come up with a blackmail plan and found a phone with an unknown number. It takes time.
Fourthly, the blackmailer behaved very cold-bloodedly, calculatingly and cruelly. I highly doubt that Delilah would act like that, given that she is emotional, and also, as you say, she wants to seduce Ashe, not blackmail her.
Fifthly, she revealed that she knew Ashe's secret not spontaneously, but intentionally. It was spontaneous there only at the very beginning, when she said that Ashe was different, and when Ashe began to ask what she meant, Delilah should have denied everything at that moment, if she were a blackmailer, but she revealed everything, intentionally. And only after that Ashe said that Fiona knew her secret. This scene is just to reveal what kind of yap Fiona is, which was predictable. Otherwise, it turns out that Delilah, if she were a blackmailer, would simply give herself away, not knowing in advance that Fiona knows everything.
I hope I didn't make a mistake with the English here.
. People have been very active! It's a lot of fun to read through all the theories you've come up with.
