How to darken an image using the Imager plugin
Seems like you might be better using colorBlend?

{% set image = block.image.first() %}
{% set transformedImage = craft.imager.transformImage(image, { 
  width: 1500,
  height: 700,
  effects: {
    colorBlend: ['rgb(0, 0, 0)', 0.4] }
  }
}) %}