Updating user profile matrix block with plugin
I believe this is the correct way to get an existing Matrix block. Found this gem after digging around in the Google group.

$block = craft()-&gt;matrix-&gt;getBlockById(blockId);


I updated my plugin code above to this and it seems to work properly.

// THIS DOESN'T WORK
// $block = new MatrixBlockModel($post['kidId']);

// THIS WORKS
$block = craft()-&gt;matrix-&gt;getBlockById($post['kidId']);

$block-&gt;fieldId    = $kidsMatrix-&gt;id; // Matrix field's ID, 'Kids'
$block-&gt;ownerId    = $currentUser-&gt;id; // ID of entry the block should be added to
$block-&gt;typeId     = $kidBlock-&gt;id; // ID of block type, 'kid'
$block-&gt;getContent()-&gt;setAttributes(array(
    'wooly' =&gt; $post['myWooly'],
));

$response = craft()-&gt;matrix-&gt;saveBlock($block);