when you read pdf by getImageBlog and want to get the image or blob of a specific page, use this function as below.
$ims = new Imagick();
$ims->readImageBlob($pdfFileContent);
$ims->setImageIndex(5); //this will return 6th page of the pdf file
$image->setImageFormat('png');
//etc
$blob = $image->getImageBlob();