Selecting Pixels based on a Blend Option

Question

I would like to select pixels based on a blend option— Multiply, Screen, etcetera— and I have yet to find out how to do this.  For example, I use the "Filter→Render→Clouds" effect, and I want to select pixels using the Screen effect so that white pixels will be at 100% opacity and black pixels will be at 0% opacity in the selection.  A plug-in would work as well.
My apologies if this isn't the correct board for this type of thing.

Answer 1

All right— I figured something out that works good enough.
Here is a step-by step of what I did: [Note: Click on each image for a full-sized preview]
First, make / open an image.  Make a backup copy if you opened an image.  I rendered a cloud effect.
Select Image → Mode → Grayscale
Open up the Channels window.
Create a new channel, it will automatically make it an Alpha channel.
Select the image and paste it into the Alpha 1 channel.
Clear the "Gray" channel to 100% black [no transparentcy] and highlight Alpha 1 and click on the select button.
Duplicate the main layer by dragging it to the new layer button and highlight the Gray channel and go to Select→ Save Selection
Save it as a Mask and choose OK
Next, to to Layer→ Layer Mask→ Apply
And there you have it! White has 0% opacity, and black has 100%!
If you wanted it to be reversed, as white has 100% opacity, and black has 0% opacity follow these steps:
Right click on the icon of the layer, and choose Select Pixels
Go to Select→ Inverse
Make a new layer and hide the layer with the black.
Fill that layer with white.
Took me quite some time, but I figured it out.  I thought I would share it with anyone who needs to use it.