Ahhhhh, ok! Thanks for the prompt reply!
Edit: Spoke to soon! There's still something up, I think. In the attached image, you'll see the results of a test. In this test, WorkingImage and TrashMaskImage are the same perlin noise image. In flash (on the left), things are working as I expected; I'm blending them using multiply and the dark parts are getting darker. In windows (on the right), nothing is changing. This is the unedited perlin noise texture... it looks exactly the same if I disable the multiply block. So, what am I missing? Do the various supported blend types actually work different on different platforms? Do they intentionally produce different results? Many thanks for any additional insight anyone can provide!