On Saturday, she ran into a group of friends she hadn't seen for a month. One of them, catching sight of the new inky blackness, was visibly shocked.
He looked at her with some concern.
'What happened?' he said. 'What made you do it?' He seemed to think she was making a wild public statement in response to some terrible private trauma.
'Which was strange,' she said to me afterwards, 'as he'd shaved a whole new design on one side of his head, and no one commented on it at all.'
So why do teenagers make dramatic changes to the way they look? Is it to turn expectations upside-down? To mark themselves out as completely different? All the old clichés about shock and rebellion?
I don't know why my daughter went out and bought a packet of ebony dye. But I have a pretty good idea.
I don't think it's the stress of waiting for GCSE results. I don't think it's anxiety about changing schools. I don't even think it's because she's got a keen interest in fashion and textiles and wants to experiment with different ways of changing her appearance.
I think it's because she's completely besotted with her kitten, who is beautiful, sleek, beguilingly affectionate – and utterly, utterly, utterly black.