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I frequently use predominately black masks on gradient or fill layers to show a shape with a color that I can choose later. However, when I transform the layer, the extension color is white- i.e. opaque. This leaves filled areas around the image where photoshop had to create more mask.
Anyway I can set this to fill new areas with black i.e. transparent?
Automatically? No.
You can simply select the areas and fill them manually. There's no automated method which I'm aware of that will arbitrarily fill non-masked areas. Photoshop, while powerful, can not make the decision of which areas are desired and which are not.
If you want to rotate the layer and layer mask, just link them (select both, and click on "Link Layers").
