Имитация затемнения

private void pictureBox6_Paint(object sender, PaintEventArgs e)
{
        Control control = (Control)sender;
        Control parent = control.Parent;
 
        Bitmap behind = new Bitmap(parent.Width, parent.Height);
 
        foreach (Control c in parent.Controls)
            if (c != control)
                c.DrawToBitmap(behind, c.Bounds);
 
        float opacity = 0.8f;
        Color bgColor = Color.FromArgb(Convert.ToInt32(255 * opacity), 0, 0, 0);
 
        Graphics g = Graphics.FromImage(behind);
        g.FillRectangle(new SolidBrush(bgColor), 0, 0, parent.Width, parent.Height);
 
        e.Graphics.DrawImage(behind, -control.Left, -control.Top);
}

 using (Graphics g = Graphics.FromImage(BackPanel))
            {
                SolidBrush Brush = new SolidBrush(Color.FromArgb(100, 0, 0, 0));
                g.FillRectangle(Brush, 0, 0, BackPanel.Width, BackPanel.Height);
            }

    private void button1_Click(object sender, EventArgs e)
    {
        // take a screenshot of the form and darken it:
        Bitmap bmp = new Bitmap(this.ClientRectangle.Width, this.ClientRectangle.Height);
        using (Graphics G = Graphics.FromImage(bmp))
        {
            G.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceOver;
            G.CopyFromScreen(this.PointToScreen(new Point(0, 0)), new Point(0, 0), this.ClientRectangle.Size);
            double percent = 0.60;
            Color darken = Color.FromArgb((int)(255 * percent), Color.Black);
            using (Brush brsh = new SolidBrush(darken))
            {
                G.FillRectangle(brsh, this.ClientRectangle);
            }
        }
 
        // put the darkened screenshot into a Panel and bring it to the front:
        using (Panel p = new Panel())
        {
            p.Location = new Point(0, 0);
            p.Size = this.ClientRectangle.Size;
            p.BackgroundImage = bmp;
            this.Controls.Add(p);
            p.BringToFront();
 
            // display your dialog somehow:
            Form frm = new Form();
            frm.StartPosition = FormStartPosition.CenterParent;
            frm.ShowDialog(this);
        } // panel will be disposed and the form will "lighten" again...
    }

using System.Drawing;
using System.Windows.Forms;
 
public class BlackoutControl : Control
{
    public BlackoutControl()
    {
        SetStyle(ControlStyles.SupportsTransparentBackColor, true);
        SetStyle(ControlStyles.Opaque, true);
        SetStyle(ControlStyles.ResizeRedraw, true);
        this.BackColor = Color.Transparent;
        this.Paint += BlackoutControl_Paint;
    }
 
    private void BlackoutControl_Paint(object sender, PaintEventArgs e)
    {
        e.Graphics.FillRectangle(new SolidBrush(Color.FromArgb(180, Color.Black)), e.ClipRectangle);
    }
 
    protected override CreateParams CreateParams
    {
        get
        {
            const int WS_EX_TRANSPARENT = 0x20;
            CreateParams cp = base.CreateParams;
            cp.ExStyle |= WS_EX_TRANSPARENT;
            return cp;
        }
    }
}