[c#] Transparent control over PictureBox

In my C# Form I have a Label that displays a download percentage in the download event:

  this.lblprg.Text = overallpercent.ToString("#0") + "%";

The Label control's BackColor property is set to be transparent and I want it to be displayed over a PictureBox. But that doesn't appear to work correctly, I see a gray background, it doesn't look transparent on top of the picture box. How can I fix this?