It sounds like you need to learn how to use a layer as a Track Matte.
Duplicate your background layer, apply blur to it. Then place your black and white matte layer above the new layer. Set the Track Matte value of the blurred layer to Luma Matte.
It sounds like you need to learn how to use a layer as a Track Matte.
Duplicate your background layer, apply blur to it. Then place your black and white matte layer above the new layer. Set the Track Matte value of the blurred layer to Luma Matte.