Removing Black Background from stock footage using add/screen blending mode not working

Agree with Rick. There appears to be no layer underneath to actually blend with, so this method won't work. The comp background doesn't count since it's treated differently. Using a Shift Channels effect to convert the luminance to transparency would be the quickest way to resolve this.

Mylenium 

Add, and Screen do not produce transparency; they only blend the pixels on the layer with values higher than zero with the pixels on the layers below. You need a layer below your stock footage before blend modes will do anything. Layers 8 and 9 are both set to add. There is nothing below them, so there are no pixels to blend with. I assume from what I see in the screenshot that both layers are stock footage of particles. If you turn off layer 8, the image will change because the pixels with a greater value than 1 on layer 8 are blended with the pixels in layer 9. Put a dark red solid below layer 9; you should see white particles on a red background. I don't think there is anything wrong.

If you want transparency, you need to create a procedural matte. You might try Set Channels and set the Alpha to use Luminance. You might need to apply a Curves effect above the Set Channels to the layer to increase the contrast of the layer to get the transparency to work the way you want it to work.