How to override block in magento module

Question

I have following structure

<?xml version="1.0"?>
<config>
<modules>
    <Namespace_News>
        <version>0.1.0</version>
    </Namespace_News>
</modules>
<frontend>
    <routers>
        <news>
            <use>standard</use>
            <args>
                <module>Namespace_News</module>
                <frontName>news</frontName>
            </args>
        </news>
    </routers>
    <layout>
        <updates>
            <news>
                <file>news.xml</file>
            </news>
        </updates>
    </layout>
</frontend>
<admin>
    <routers>
        <news>
            <use>admin</use>
            <args>
                <module>Namespace_News</module>
                <frontName>news</frontName>
            </args>
        </news>
        <adminhtml>
            <args>
                <modules>
                    <namespace_news before="Mage_Adminhtml">Namespace_News</namespace_news>
                </modules>
            </args>
        </adminhtml>
    </routers>
</admin>
<adminhtml>
    <menu>
        <news module="news">
            <title>News</title>
            <sort_order>71</sort_order>
            <children>
                <items module="news">
                    <title>Manage Items</title>
                    <sort_order>0</sort_order>
                    <action>news/adminhtml_news</action>
                </items>
                <account module="news">
                    <title>My Account</title>
                    <sort_order>1</sort_order>
                    <action>news/system_account</action>
                </account>
            </children>
        </news>
    </menu>
    <acl>
        <resources>
            <all>
                <title>Allow Everything</title>
            </all>
            <admin>
                <children>
                    <Namespace_News>
                        <title>News Module</title>
                        <sort_order>10</sort_order>
                    </Namespace_News>
                    <Namespace_Account>
                        <title>My Account</title>
                        <sort_order>1</sort_order>
                    </Namespace_Account>
                </children>
            </admin>
        </resources>
    </acl>
    <layout>
        <updates>
            <news>
                <file>news.xml</file>
            </news>
        </updates>
    </layout>
</adminhtml>   
<global>
    <models>
        <news>
            <class>Namespace_News_Model</class>
            <resourceModel>news_mysql4</resourceModel>
        </news>
        <news_mysql4>
            <class>Namespace_News_Model_Mysql4</class>
            <entities>
                <news>
                    <table>news</table>
                </news>
            </entities>
        </news_mysql4>
    </models>
    <resources>
        <news_setup>
            <setup>
                <module>Namespace_News</module>
            </setup>
            <connection>
                <use>core_setup</use>
            </connection>
        </news_setup>
        <news_write>
            <connection>
                <use>core_write</use>
            </connection>
        </news_write>
        <news_read>
            <connection>
                <use>core_read</use>
            </connection>
        </news_read>
    </resources>
    <blocks>
        <news>
            <class>Namespace_News_Block</class>
        </news>
        <adminhtml>
             <rewrite>
        <system_account_edit>Namespace_News_Block_Account_Edit</system_account_edit>
<system_account_edit_form>Namespace_News_Block_Account_Edit_Form</system_account_edit_form>
            </rewrite>
        </adminhtml>
    </blocks>
    <helpers>
        <news>
            <class>Namespace_News_Helper</class>
        </news>
    </helpers>
</global>

I have Form.php at directory
app/code/local/Namespace/News/Block/Account/Edit/Form.php

<?php
//require_once "Mage/Adminhtml/Block/System/Account/Edit/Form.php";
class Namespace_News_Block_Account_Edit_Form extends Mage_Adminhtml_Block_System_Account_Edit_Form
{
    protected function _prepareForm()
    {
        $userId = Mage::getSingleton('admin/session')->getUser()->getId();
        $user = Mage::getModel('admin/user')->load($userId);
        $user->unsetData('password');
        $form = new Varien_Data_Form();
        $fieldset = $form->addFieldset('base_fieldset', array('legend'=>Mage::helper('adminhtml')->__('Account Information')));
        $fieldset->addField('username', 'text', array(
                'name'  => 'username',
                'label' => Mage::helper('adminhtml')->__('User Name1'),
                'title' => Mage::helper('adminhtml')->__('User Name'),
                'required' => true,
            )
        );
        $fieldset->addField('firstname', 'text', array(
                'name'  => 'firstname',
                'label' => Mage::helper('adminhtml')->__('First Name1'),
                'title' => Mage::helper('adminhtml')->__('First Name'),
                'required' => true,
            )
        );
        $fieldset->addField('lastname', 'text', array(
                'name'  => 'lastname',
                'label' => Mage::helper('adminhtml')->__('Last Name'),
                'title' => Mage::helper('adminhtml')->__('Last Name'),
                'required' => true,
            )
        );
        $fieldset->addField('user_id', 'hidden', array(
                'name'  => 'user_id',
            )
        );
        $fieldset->addField('email', 'text', array(
                'name'  => 'email',
                'label' => Mage::helper('adminhtml')->__('Email'),
                'title' => Mage::helper('adminhtml')->__('User Email'),
                'required' => true,
            )
        );
        $fieldset->addField('password', 'password', array(
                'name'  => 'new_password',
                'label' => Mage::helper('adminhtml')->__('New Password'),
                'title' => Mage::helper('adminhtml')->__('New Password'),
                'class' => 'input-text validate-admin-password',
            )
        );
        $fieldset->addField('confirmation', 'password', array(
                'name'  => 'password_confirmation',
                'label' => Mage::helper('adminhtml')->__('Password Confirmation'),
                'class' => 'input-text validate-cpassword',
            )
        );
        $form->setValues($user->getData());
        $form->setAction($this->getUrl('*/system_account/save'));
        $form->setMethod('post');
        $form->setUseContainer(true);
        $form->setId('edit_form');
        $this->setForm($form);
        return parent::_prepareForm();
    }
}

I am trying to override block Form.php but it doesn't work. Thanks in advance.

Answer 1

<blocks>
    <news>
        <class>Xoptimo_News_Block</class>
    </news>
    <adminhtml>
        <rewrite>
            <system_account_edit_form>Xoptimo_News_Block_Account_Edit_Form</system_account_edit_form>
        </rewrite>
    </adminhtml>
</blocks>

and Please Check your all Namespace should replace with the Xoptimo