Adding target and _blank as it's value to the allowedtags

I know that WordPress uses KSES to parse HTML and strip invalid XHTML so I took control of $allowedtags in my themes functions.php file inside a function then added 'target' keyed to an array which contains '_blank' keyed to an empty array. I thought this would work but my link still seems to not contain the target attribute? I have tried re saving the HTML also.

Here is my code:

function add_required_html_tags() 
{
    global $allowedtags; //tap into wordpress global array of allowed tags
    //$target
    //$allowedtags[] = 'target';
    $allowedtags['target'] = array('_blank'=array()); //add target html tag in the style that wordpress expects
    //var_dump($allowedtags);
}
add_action('init', 'add_required_html_tags', 10); //add action to initialise so we can access this tag everywhere.

and here is the result of a var_dump

array(15) { ["a"]= array(2) { ["href"]= bool(true) ["title"]= bool(true) } ["abbr"]= array(1) { ["title"]= bool(true) } ["acronym"]= array(1) { ["title"]= bool(true) } ["b"]= array(0) { } ["blockquote"]= array(1) { ["cite"]= bool(true) } ["cite"]= array(0) { } ["code"]= array(0) { } ["del"]= array(1) { ["datetime"]= bool(true) } ["em"]= array(0) { } ["i"]= array(0) { } ["q"]= array(1) { ["cite"]= bool(true) } ["s"]= array(0) { } ["strike"]= array(0) { } ["strong"]= array(0) { } ["target"]= array(1) { ["_blank"]= array(0) { } } } 

(the target element is at the end) Can anyone see where I am going wrong? I also tried changing _blank to a bool with the value of true. - no luck there.

Using wordpress 4.4.2

UPDATE:

code is now:

function add_required_html_tags() 
{
    global $allowedposttags; //tap into wordpress global array of allowed tags
    global $allowedtags; //tap into wordpress global array of allowed tags
    //$target
    //$allowedtags[] = 'target';
    $allowedtags['a'] = array('href' = true, 'title' = true, 'target' = true);
    $allowedposttags['a'] = array('href' = true, 'title' = true, 'target' = true); //add target html tag in the style that wordpress expects
    //var_dump($allowedtags);
}
add_action('init', 'add_required_html_tags', 10); //add action to initialise so we can access this tag everywhere.

The target is an attribute of the a tag so I've added it as an allowed attribute but it still isn't working?