(PECL imagick 2.0.0)
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
   Returns TRUE on success.
  
| Version | Description | 
|---|---|
| 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();
}
?>