Can't think of any way to take this to the gutter, girls, sorry!
It sounds like "darken" - so I am going with "to get darker". Of course, it also sounds like "sparkle".... to sparkle darkly?
That's right Cara you just go ahead and play innocent. With the picture of the bush you just posted no one will believe you!
Oh Lordy... we're talking about Cara's bush again! LOL
Darkle: To become clouded or gloomy. To grow dark. To become concealed in the dark.