How do I tell hook_block() about placement
You should use hook_block_info_alter() instead of hook_block(). For example:

function YOURMODULE_block_info_alter(&amp;$blocks, $theme, $code_blocks) {
  $blocks['MODULE']['BLOCK']['visibility'] = BLOCK_VISIBILITY_LISTED;
  $blocks['MODULE']['BLOCK']['pages'] = 'user/* \n stream';
}


Where YOURMODULE is the name of your module, MODULE is the module generating the block, and BLOCK is the name of the block. You should list all paths where the block should be visible separated by \n, \r or \n\r.