Are you being specific about the word darken? Because LCDs are design to either allow or block light. So technically an LCD can "darken" incoming light. But I think the point of inconsistent lighting from the environment would be the biggest challenge.
The best example of transparent display I've seen in production is:https://glas.johnsoncontrols.com/
But the material on that seems darker and I believe they use an edge light to provide higher contrast.
I think the distance from the eye, as well, would be difficult if you aren't doing some type of projection, like what is used in the the article.
