I dont know if I understand what exactly you want, but here is something that should help you.
1- Add a new form to your project "Form 2"
2- Then you have two forms, so call the form number two through form number one.
Form2 randomname = new Form2();
randomname.ShowDialog();
3-Then in the Form2 Load you can control the transparency level usin this:
private void Form2_Load(object sender, EventArgs e)
{
this.Opacity = 0.5;
}
Hope this help.