Creative Image Editing in Photoshop - Black and White in Photoshop CS4 and Photoshop Lightroom - page 229

Blurring the Lab “B” channel - The method
Step 1: Go to the Image Menu Mode Lab Color.
Step 2: Activate the Channels palette and click the “b” channel to make it
active.
Step 3: Go to the Filter Menu Noise Dust and Scratches. Move the Radius
until you blur the chunky look of the noise out of the image. Click OK. The “b”
channel is where the majority of your noise will be.
Step 4: Do the same to the “a” channel, but to a lesser degree.
Step 5: Click back to the Image Menu Mode, and change the image back
to RGB.
Another quick method: try converting your image to a smart i lter. Filter
Menu Convert to Smart Filter. Then choose Filter Noise Reduce Noise.
Fade and edit the i lter to suit the image.