C# Definition:

[StructLayout(LayoutKind.Sequential)]
struct DWM_BLURBEHIND
{
    public DWM_BB dwFlags;
    public bool fEnable;
    public IntPtr hRgnBlur;
    public bool fTransitionOnMaximized;

    public DWM_BLURBEHIND(bool enabled)
    {
        fEnable = enabled ? 1 : 0;
        hRgnBlur = IntPtr.Zero;
        fTransitionOnMaximized = 0;
        dwFlags = DWM_BB.Enable;
    }

    public System.Drawing.Region Region
    {
        get { return System.Drawing.Region.FromHrgn(hRgnBlur); }
    }

    public bool TransitionOnMaximized
    {
        get { return fTransitionOnMaximized > 0; }
        set
        {
            fTransitionOnMaximized = value ? 1 : 0;
            dwFlags |= DWM_BB.TransitionOnMaximized;
        }
    }

    public void SetRegion(System.Drawing.Graphics graphics, System.Drawing.Region region)
    {
        hRgnBlur = region.GetHrgn(graphics);
        dwFlags |= DWM_BB.BlurRegion;
    }

}

VB.NET Definition:

<StructLayout(LayoutKind.Sequential)> _
Structure DWM_BLURBEHIND
    Public dwFlags As DWM_BB
    Public fEnable As Boolean
    Public hRgnBlur As IntPtr
    Public fTransitionOnMaximized As Boolean
End Structure

User-Defined Field Types:

DWM_BB

Notes:

None.