I'm new to Illustrator, so I'm not sure if this is an easy or hard thing to do. I'll explain the process I want to do...
I have multiple layers of the same image, on one copy of the image I have used the 'poster edges' filter and I now want to cut just the black color from this and paste onto a new layer leaving me with just the outline of the picture that 'poster edges has created.
Any help or thoughts would be greatly appreciated :)
Try all of the different blending modes (Transparency palette), and see if any will suit your needs.
You could use the pen tool to draw a Clipping Mask(s)on top of the area that you want to isolate. Note: If there are multiple disconnected masks you will need to select all of them and Object/Compound Path/Make, prior to creating the Clipping Mask. These masks must each be closed paths. Make sure the mask is above the target image.
Than, select the target image and your new Clipping Mask(s) and Object/Clipping Mask/Make. Boom!
GENERATE A MASK IN PHOTOSHOP:
If you need to create a complex mask, or don't want to spend time doing a detailed drawing in Illustrator, you can open the image in Photoshop, use the Wand tool to to make your selection.
Open the Paths palette and click the 'Make work path from selection' button, than click the drop down (triangle) button and select 'Save Path'.
Finally, choose File/Export/Paths to Illustrator.
Now, you can copy the vector out of this new file and paste it into your composition to use as your clipping mask.
There are also Transparency Mask options in Illustrator, which might be more appropriate for your job - but they are more complicated, so let's try the Clipping Mask option first.