Start with new image, and fill it with very dark blue color. Then, with very large (800-1000 px) Soft Round Brush, add a bit of black here and there. Then, apply blur (filters > blur > gaussian > 30-40 px) to smooth the background.

The following steps will help us to create insteresting non-homogeneous texture. Create New Layer, switch to Rough Round Bristle brush, set color to white, and fill entire layer with the chaotic brush strokes.

Then, set this layer blend mode to Overlay. It will look almost invisible, but after all this is only a background :-)

In another new layer, repeat the same but with black brush.