I have been trying to create a BlendState for my UI text sprites so that they are both alpha-blended (so you can see them) and invert the pixel they are rendered over (again, so you can see them).
In order to get alpha blending you would need:
SrcBlend = SRC_ALPHA
DestBlend = INV_SRC_ALPHA
and in order to have inverted colours you would need something like:
SrcBlend = INV_DEST_COLOR
DestBlend = INV_SRC_COLOR
and you can't have both.
So I have come to the conclusion that it's not possible; am I right?