The documentation says about Color.FromNonPremultiplied Method: Converts a non-premultipled alpha color to a color that contains premultiplied alpha.
Based on this you should use either
BlendState.NonPremultiplied
and Color(12, 34, 56, 78)
OR
BlendState.AlphaBlend
and Color.FromNonPremultiplied(12, 34, 56, 78)
together.