How can I set the background as black color transparency using c#.net

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C#

Thanks in advance....I have a picturebox at back ground image and I need to open a Pop over the picturebox.....If click the button automatically the background picturebox gets black transparency and opens the Popup...How can I set the Transparency to picturebox backcolor black.

What I have tried:

pictureBox2.BackColor = Color.Black;
           GrpSupport.Visible = true;
           GrpSupport.BringToFront();