Per the hook_block_info()
docs, you need to add visibility
:
array(
'module' => 'my_custom_module',
'delta' => 'my_block',
'theme' => $default_theme,
'status' => 1,
'weight' => 0,
'region' => 'help',
'pages' => "admin/page1\nadmin/page2\nadmin/page3",
'cache' => 0,
'visibility' => BLOCK_VISIBILITY_LISTED
),