Invalid Credentials error when trying to add post to blogger using API

I have trying to add a post to blogger using API since yesterday but no 
luck. Here is my code. When I run it, I get "Invalid Credentials" error.

I have also posted screenshot below which shows from where I am getting 
that key in Google Developer Console. May be I am picking up wrong key?

*Code*

<?php
$key  = "J7c";
$blog_id = "123456";
$url = 'https://www.googleapis.com/blogger/v3/blogs/'.$blog_id.'/posts/';
$postData = array(
    'kind' => 'blogger#post',
    'blog' => array('id' => $blog_id),
    'title' => 'This is title',
    'content' => 'This is content'
);
$data_string = json_encode($postData);
$head = array();
$head[] = 'Authorization: '.$key;
$head[] = 'Content-Type: application/json';
$ch = curl_init();
curl_setopt($ch, CURLOPT_HTTPHEADER, $head);
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
$rsp = curl_exec($ch);
$results = json_decode($rsp);
var_dump($rsp);
?>


*Error*

<http://i.stack.imgur.com/mkg9R.png>


*Google Developers Console*

<http://i.stack.imgur.com/I85wi.png>


 

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