It's not foreshadowing - it's character development.
Dawn was foreshadowed - 'Graduation Day', 'This Year's Girl', 'Restless', there's even some speculation that Michelle Tractenberg was actually IN 'Restless'.
Willow - in this instance - is very different.
This is moment where we see Willow REALLY start to change, where she disregards everything apart from her needs, and in this case, it was her need to take vengeance on Glory for what happened to Tara. She was willing to delve into some dark, dark magics in order to get the power she needed to really hurt Glory.
It was a warning shot - Look, look at Willow, look how far she's willing to go to for Tara, this is some hardcore stuff. She's not going to be able to come back from that, is she? Oh, wait, she's okay. She's looking after Tara, she's going to keep Tara safe, she' okay. Isn't she?
I think we had to see Willow go dark before S6 in order for us to accept that phase of her life. We saw those black eyes and know that she had that in her, but we also saw her come back from it, to turn away from it, so we knew that it was a choice Willow made. Yes, she was consumed with grief, but it was still a choice that Willow made to turn to the darker magic to do what she wanted, and seeing her turn away from it before was always in the back of our minds when we were watching S6, so we had hope that she would be able to do it again.