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Perhaps what you are remembering are "layer masks".  Google {photoshop "layer masks"}.

Where the mask for a layer is black, that layer won't contribute to the final image, whereas if it's white in some area, it will contribute 100% in that area. 

So, one way to darken a specific area of an image is to put a "curves adjustment layer" at the top of your layer stack, make the curve lay substantially below the diagonal, and manually paint white into those portions of the image that you want to darken.

There are lots and lots of other ways to have a similar effect.  Perhaps others will chime in with their favorite techniques.

HTH,

Tom M