General developer forum

For the current page, use $PAGE->blocks.

To see what is on other pages, you need code like this:

$page = new \moodle_page();
$page->set_url('/course/view.php', array('id' => $this->course->id));
$page->set_pagelayout('course');
$page->set_course($this->course);

$context = \context_course::instance($this->course->id);

$blockmanager = $page->blocks;

$blockmanager->load_blocks(true);

$missingblocks = self::$BLOCKS_TO_ENSURE_EXIST;
$desiredblockpositions = self::$BLOCK_ORDER;
$targetblockpositions = array();
foreach ($blockmanager->get_regions() as $region) {
    foreach ($blockmanager->get_blocks_for_region($region) as $block) {
        $instance = $block->instance;

        if ($instance->parentcontextid == $context->id && in_array($instance->blockname, self::$BLOCKS_TO_REMOVE)) {
            blocks_delete_instance($instance);
            $this->log_action('blockremoved', $this->block_type_name($instance->blockname));
            continue;
        }

        if (isset($missingblocks[$instance->blockname])) {
            unset($missingblocks[$instance->blockname]);
        } else {
            if (isset($desiredblockpositions[$instance->blockname])) {
                $tempname = $instance->blockname;
            } else {
                $tempname = 'OTHER';
            }
            $targetblockpositions[$instance->id] =
                    array($instance, $desiredblockpositions[$tempname]);
            $desiredblockpositions[$tempname] += 1;
        }
    }
}