but I have no clue on how to render everything except the SKY black into a texture
Very easy do this, just do far negative fog, like this
technique black
{
pass p0
{
AlphaBlendEnable=True;
Zenable = True;
ZwriteEnable = True;
BlendOp=Add;
Lighting=False;
FogEnable = True;
FogColor = float4(0.0f, 0.0f, 0.0f, 1.0f);
FogStart = 60000;
FogEnd = 50000;
ShadeMode = Flat;
ColorArg1[0] = Diffuse;
ColorOp[0] = SelectArg1;
}
}
set this material to view