So we all know that Avacyn now has black eyes and a shifted spear, and the artwork goes further by granting her a fully blackened spear and a few more evil looking details.

However, in "A Blank Gaze", the eyes and spear shape are the only changes. In fact, the latter is outrightly described as still glowing, albeit in "red and gold", as opposed to the cold or black appearence it now has.

I find this artistic choices, while not surprising, a bit odd. They seem to imply even that Avacyn doesn't even have magical powers anymore, when that's not the case.

Maybe she's still degenerating and hasn't hit the level of Purifier's art yet?

What I want to know is who died to incite her change.
That's literally her trigger on the card.

Zendikar.

Perhaps, but the text seems to imply that she reached her present form. At any rate it stopping to glow seems like a weird change.