ImageMagick PHP | Dynamic conversion from RGB to CMYK

Question

I'm trying to make an algorithm that converts an image from RGB to CMYK and that modifies each CMYK channel with a given percentage. For example: I upload my RGB image and I say, convert it to CMYK and add 70% of magenta. The percentage can be negative too. Now, by using PHP ImageMagick I've done this so far:

     if ($imageMagick) {
        $pixelIter = $imageMagick->getPixelIterator();
        foreach($pixelIter as $row) {
          foreach($row as $pixel) {

            $cyanValue = $pixel->getColorValue(Imagick::COLOR_CYAN);
            $magentaValue = $pixel->getColorValue(Imagick::COLOR_MAGENTA);
            $yellowValue = $pixel->getColorValue(Imagick::COLOR_YELLOW);
            $blackValue = $pixel->getColorValue(Imagick::COLOR_BLACK);

            $cyanPercent = ($cyanValue / 100) * abs($cyan);
            $magentaPercent = ($magentaValue / 100) * abs($magenta);
            $yellowPercent = ($yellowValue / 100) * abs($yellow);
            $blackPercent = ($blackValue / 100) * abs($black);
            
            if ( $cyan < 0 ) {
                $finalCyan = $cyanValue - $cyanPercent;
            } else {
                $finalCyan = $cyanValue + $cyanPercent;
            }
            
            if ( $magenta < 0 ) {
                $finalMagenta = $magentaValue - $magentaPercent;
            } else {
                $finalMagenta = $magentaValue + $magentaPercent;
            }
            
            if ( $yellow < 0 ) {
                $finalYellow = $yellowValue - $yellowPercent;
            } else {
                $finalYellow = $yellowValue + $yellowPercent;
            }
            
            if ( $black < 0 ) {
                $finalBlack = $blackValue - $blackPercent;
            } else {
                $finalBlack = $blackValue + $blackPercent;
            }
            
            $pixel->setColorValue(Imagick::COLOR_CYAN, $finalCyan);
            $pixel->setColorValue(Imagick::COLOR_MAGENTA, $finalMagenta);
            $pixel->setColorValue(Imagick::COLOR_YELLOW, $finalYellow);
            $pixel->setColorValue(Imagick::COLOR_BLACK, $finalBlack);
          }
          $pixelIter->syncIterator();
        }
        
        $imageMagick->writeImage("cmykImage.jpg");
        
        echo "<img src='cmykImage.jpg'>";

    }

Values of channels like $cyan and $magenta are taken in input with $_POST and they are an integer between -100 and 100.

This code works pretty good. The conversion works, the first three channels are changed correctly. The only problem is that the black value doesn't seem to have the same behaviour as the other three channels. If I try to change only the black, the image remain the same, even though the code is identical for all the channels. Any ideas?

Another thing I tried: every iteration, I tried to calculate the correct color of the pixel and set it with the setColor() function, but in the end I obtain a pixelated image, which is not what I want.

Thank you so much.