block.php

$block = module_invoke('module', 'block_view', 'delta');
print render($block['content']);

block_load_through_template.php

$block = block_load('multiblock', 3);
$render_array = _block_get_renderable_array(_block_render_blocks(array($block)));
$output = render($render_array);
print $output;

block_visibility_by_active_menu_item.php

if (arg(0) == 'node' && is_numeric(arg(1))) {
  $active_trail = menu_get_active_trail();
  end($active_trail);
  $nid = 1; // change to nid of parent node you want to check
  if ($active_trail[1] && $active_trail[1]['link_path'] == 'node/'.$nid) {
    return true;
  } else {
    return false;
  }
}