You need to pass this function a colour:
<?php
$img->blackThresholdImage( "#FFFFFF" );
?>
E.g. this blackens any pixel which isn't pure white.(PECL imagick 2, PECL imagick 3)
Imagick::blackThresholdImage — Forces all pixels below the threshold into black
Is like Imagick::thresholdImage() but forces all pixels below the threshold into black while leaving all pixels above the threshold unchanged.
thresholdThe threshold below which everything turns black
   Restituisce true in caso di successo.
  
| Versione | Descrizione | 
|---|---|
| PECL imagick 2.1.0 | Now allows a string representing the color as a parameter. Previous versions allow only an ImagickPixel object. | 
Example #1 Imagick::blackThresholdImage()
<?php
function blackThresholdImage($imagePath, $thresholdColor) {
    $imagick = new \Imagick(realpath($imagePath));
    $imagick->blackthresholdimage($thresholdColor);
    header("Content-Type: image/jpg");
    echo $imagick->getImageBlob();
}
?>You need to pass this function a colour:
<?php
$img->blackThresholdImage( "#FFFFFF" );
?>
E.g. this blackens any pixel which isn't pure white.Here's a example of this function:
<?php
  $img = new Imagick();
  $img->readImage($image_file_name);
  $img->blackThresholdImage('grey');
  $img->writeImage($thumb_file_name);
  $img->clear();
  $img->destroy();
?>