magic
Well, you could say that there is a bit of "magic" since Blake naturally meets plenty of physical conditions for her transition to womanhood to be very optimal xD. But this also happens to some real people, so it's not impossible.
Blake has dense hair, a rounded hairline, is beardless, has almost no body hair, her facial features are soft, her tone of voice is fairly neutral/high, and her body is reasonably curvy. And all of this is already the case since the beginning, when Blake still identifies as a boy.
The most boyish feature Blake has is her flat chest, but Blake is OK with that, and it's not like there aren't cis girls with fairly flat chests either. Her penis is also small, which makes Blake feel more feminine.
Blake also knows how to do her hair, makeup and dress to enhance her natural feminine features. For example, she avoids wearing clothes that require big breasts to look good, since they wouldn't look good on her.
Blake is simply very lucky with her genetics, and she also knows how to take advantage of her natural features.