Blur an image with c#. More information: http://eric.tumblr.com/post/71460344489/blur-an-image-with-c

Blur.cs

private static Bitmap Blur(Bitmap image, Rectangle rectangle, Int32 blurSize)
{
    Bitmap blurred = new Bitmap(image.Width, image.Height);
 
    // make an exact copy of the bitmap provided
    using(Graphics graphics = Graphics.FromImage(blurred))
        graphics.DrawImage(image, new Rectangle(0, 0, image.Width, image.Height),
            new Rectangle(0, 0, image.Width, image.Height), GraphicsUnit.Pixel);
 
    // look at every pixel in the blur rectangle
    for (Int32 xx = rectangle.X; xx < rectangle.X + rectangle.Width; xx++)
    {
        for (Int32 yy = rectangle.Y; yy < rectangle.Y + rectangle.Height; yy++)
        {
            Int32 avgR = 0, avgG = 0, avgB = 0;
            Int32 blurPixelCount = 0;
 
            // average the color of the red, green and blue for each pixel in the
            // blur size while making sure you don't go outside the image bounds
            for (Int32 x = xx; (x < xx + blurSize && x < image.Width); x++)
            {
                for (Int32 y = yy; (y < yy + blurSize && y < image.Height); y++)
                {
                    Color pixel = blurred.GetPixel(x, y);
 
                    avgR += pixel.R;
                    avgG += pixel.G;
                    avgB += pixel.B;
 
                    blurPixelCount++;
                }
            }
 
            avgR = avgR / blurPixelCount;
            avgG = avgG / blurPixelCount;
            avgB = avgB / blurPixelCount;
 
            // now that we know the average for the blur size, set each pixel to that color
            for (Int32 x = xx; x < xx + blurSize && x < image.Width && x < rectangle.Width; x++)
                for (Int32 y = yy; y < yy + blurSize && y < image.Height && y < rectangle.Height; y++)
                    blurred.SetPixel(x, y, Color.FromArgb(avgR, avgG, avgB));
        }
    }
 
    return blurred;
}

BlurOverloaded.cs

private static Bitmap Blur(Bitmap image, Int32 blurSize)
{
    return Blur(image, new Rectangle(0, 0, image.Width, image.Height), blurSize);
}

update on
// now that we know the average for the blur size, set each pixel to that color
for (Int32 x = xx; x < xx + blurSize && x < image.Width && x < rectangle.Width; x++)
for (Int32 y = yy; y < yy + blurSize && y < image.Height && y < rectangle.Height; y++)
blurred.SetPixel(x, y, Color.FromArgb(avgR, avgG, avgB));

will be
// now that we know the average for the blur size, set each pixel to that color
for (int x = xx; x < xx + blurSize && x < image.Width && x < rectangle.X + rectangle.Width; x++)
for (int y = yy; y < yy + blurSize && y < image.Height && y < rectangle.Y + rectangle.Height; y++)
blurred.SetPixel(x, y, Color.FromArgb(avgR, avgG, avgB));

x < rectangle.Width replaced with rectangle.X + rectangle.Width
and y < rectangle.Height reaplced with rectangle.Y + rectangle.Height
to blur the any specific portion on the image as the code is working right for entire image but for part of image it amy or may not work depending on provided coordinates in rectangle in the below function.

return Blur(image, new Rectangle(0, 0, image.Width, image.Height), blurSize);