Darkening a blue sky without using Secondary Color

Hi
Attempting to do all my CC from with PPro now that we have adjustment layers (though may resort to exporting to Davinci...probably never Speedgrade judging from the interface)

Problem I'm having is darkening blue skyies in some shots.

I can't get an accurate enough key using secondary color in RGB curves, the edges always jump out.

Are there any other 'trick's to do this, perhaps by manipulating channels, adjustment layers and-or blend modes?

Local is the new global