How do I unhook commerce_kickstart_blog_preprocess_node?
$implementations is an array whose keys are the short name of the enabled modules; $hook is the name of the hook, without hook_ nor the short name of the module.

You need to change your code to the following one.

function my_module_implements_alter(&amp;$implementations, $hook) {
  if ($hook == 'preprocess_node' &amp;&amp; isset($implementations['commerce_kickstart_blog'])) {
   unset($implementations['commerce_kickstart_blog']);
  }
}


I assume the short name of your module is my; if it were my_module, then you would need to use the following code, since the hook is hook_module_implements_alter().

function my_module_module_implements_alter(&amp;$implementations, $hook) {
  if ($hook == 'preprocess_node' &amp;&amp; isset($implementations['commerce_kickstart_blog'])) {
   unset($implementations['commerce_kickstart_blog']);
  }
}