The darkening sky.    From Wordnet, Princeton University. 

Verb (used with object) : He darkened the festivities by his presence.    From Dictionary.com. 

Lara narrowed her eyes, her expression darkening drastically.    From Wordnik.com.  [Wraithbait] Reference

Marco formed his own conclusions, his expression darkening some more.    From Wordnik.com.  [The Bellini Bride]

Her expression darkening, Allegra began to back toward the mouth of the corridor.    From Wordnik.com.  [One Night Of Scandal]

Then, his expression darkening, he said: "I wouldn't want anything to happen to you.".    From Wordnik.com.  [The Thief of Always]

That is why, "Medan added, his expression darkening," 1 do my best to keep this land safe from the wrath of the dragon.    From Wordnik.com.  [Dragons of a Fallen Sun]

He stared at her, a puzzled expression darkening his face.    From Wordnik.com.  [Laying His Claim]

It's generally harmless but can cause permanent skin darkening.    From Wordnik.com.  ['Toasted Skin Syndrome' Can Be Caused By Warm Laptops, Researchers Warn] Reference

Frederick, an expression darkening his eyes which his mother dreaded.    From Wordnik.com.  [Tess of the Storm Country] Reference

He glanced between her and the two detectives, his own expression darkening with uncertainty and suspicion.    From Wordnik.com.  [Nevermore] Reference

After-cooking darkening occurs when potatoes are exposed to air after cooking, including boiling, baking, frying, or dehydration.    From Wordnik.com.  [EurekAlert! - Breaking News] Reference

Brad asked, his expression darkening.    From Wordnik.com.  [Nevermore] Reference

Count me among the camp that sees no point in "darkening" Mary Marvel.    From Wordnik.com.  [Archive 2008-05-01] Reference

3, 17: Luke added to this account the cause of the darkness, that is, the darkening of the sun.    From Wordnik.com.  [Catena Aurea - Gospel of Mark] Reference

I've consumed popular culture for the last 30 to 35 years, and I've noticed a very discernable "darkening" of the message.    From Wordnik.com.  [Do virtual worlds liberate us?] Reference

In this first stage of the after-life, then, there is a kind of darkening of the memory of the past earth life -- darkening, not oblivion.    From Wordnik.com.  [Reincarnation A Study in Human Evolution] Reference

When outside, look for signs of a developing thunderstorm such as darkening skies, flashes of lightning or increasing wind.    From Wordnik.com.  [Local News from Gadsden Times] Reference

One consequence of this (but not the one I'm concerned with) is the retroactive "darkening" of superhero comics from bygone days.    From Wordnik.com.  [Comic Book Resources]

Still, Moody's Investors Service said Thailand's credit rating fundamentals are "darkening" amid the escalating political tensions.    From Wordnik.com.  [Elections - fresh news by plazoo.com] Reference

The country's credit rating fundamentals are "darkening" amid escalating political protests, Moody's Investors Service said on April 9.    From Wordnik.com.  [BusinessWeek.com --] Reference

 He's standing by the darkening window, his back turned.    From Wordnik.com.  [Monsoon] Reference

Victor bled venom onto his lovely satin vest, darkening the fabric.    From Wordnik.com.  [One Thousand Incarnations and One Thousand Deaths - Part II] Reference

Outside, clouds lost their contours, darkening into a menacing murk.    From Wordnik.com.  [Into the Eye] Reference

All night the clouds had been gathering together, darkening, plotting.    From Wordnik.com.  [Water] Reference

But for now, let's look on the bright side of the darkening days ahead.    From Wordnik.com.  [What’s Hot For the Big Chill of Fall] Reference

  The night was darkening with heavy clouds that moved rapidly across the sky.    From Wordnik.com.  [THE TICK IS FULL] Reference

But U.S. carriers seem to see their skies darkening, even as the economy brightens.    From Wordnik.com.  [Our Fearful Fliers] Reference

When I felt like my mood was darkening too much to put up with conversations, I stood up.    From Wordnik.com.  [she has one of those names that only a southern girl could pull off] Reference

But if we do that, we are darkening the doorstep of the future and punishing our children.    From Wordnik.com.  [Taking Stock] Reference

  The room is darkening around them and she is sitting close enough to him to feel warmth.    From Wordnik.com.  [Post] Reference

Ignited at dusk, it unloosed a spectacular line of fire, snaking across the darkening landscape.    From Wordnik.com.  [Pop Goes the Easel] Reference

As the cloud passed over the retreating sun the effect was counter-intuitive: a darkening of its form.    From Wordnik.com.  [Pros and Cons of Wildflower Collection] Reference

 She had painted the ceiling as a darkening evening sky punctuated with two tiny pale planets and a star.    From Wordnik.com.  [And Then: Parts 1-3] Reference

Dawn was already approaching — a thin and dilute orange band against a smock of darkening grey on the horizon.    From Wordnik.com.  [Excerpt from Pompeii ii] Reference

 He shouted, but he was already three years old in this depiction, his hair longer than I remembered, darkening.    From Wordnik.com.  [The Time Broker] Reference

  Her white-walled kitchen felt empty, the darkening sky staring in, and she walked over to pull a CD from her shelf before beginning.    From Wordnik.com.  [Caitlin in the Y2K Museum] Reference