For who want to show "animated" gif. This getImageBlob() will showing only one frame of image.
Use getImagesBlob() instead.

Example:

<?php
$Imagick = new \Imagick($real_source_image_path_animated_gif);
header("Content-Type: image/gif");
echo $Imagick->getImagesBlob();
?>

Function returns NULL without any error if ImageMagick policy.xml blocks handling the file format, i.e. PDF.
For PDF output edit /etc/ImageMagick-6/policy.xml and remove the line which disables rights for PDF.
If you use other functions, e.g. readImage() for PDF, ImageMagick nags about it and outputs the error message but this function fails silently.

It seems that this method can return a zero-length string if faced with large-ish data.  No exceptions are thrown.

<?php
  $image = new Imagick();
  if (!$image->newImage(1000,1,'white')) throw new Exception();
  if (!$image->scaleImage(0,200)) throw new Exception();
  print "Image size: {$image->getImageWidth()},{$image->getImageHeight()}\n";
  if (!$image->setImageFormat("jpeg")) throw new Exception();
  $a = $image->getImageBlob();
  print "Rendered to ".strlen($a)." bytes\n";
?>

Restrict your output image size, or ensure that the blob you get back isn't empty.  (Note that IM seems to be doing the work, it delays for some time.  But there's no indication of any error anywhere.)

Further to Trevor's note above about getImageBlob potentially returning an empty string for large images.  

This seems image format dependent.  I have a large (12046x8363) image which is returned ok.  But if I call setImageFormat('pdf') and then call getImageBlob() again.  Example:

<?php
// Image Manipulation here.
$pngData = $imagick->getImageBlob();  // Returns a large PNG.
$imagick->setImageFormat('pdf');
$pdfData = $imagick->getImageBlob(); // Returns empty string immediately
?>