Gaussian blur not working when I change image to black

[Nadia]

Hi there,

I'm trying to create a shadow for my photo, so I copied and pasted it as a new layer and coloured it black. Normally I just select gaussian blur and it changes it to a shadowier image, but for some reason it's just not working. If I keep it as a pasted layer without turning it black, the Gaussian filter works. I've tried applying the filter, then colouring it black but it just goes back to a solid block colour. 

Any advice would be massively appreciated! 

Thanks,

Nadia

[Ofnuts]

I the image color-indexed? How is the layer "turned black"?