If you have a requirement to display a block in your custom page or form in drupal 8 it can be accomplished using block entity objects. First you need to get the block id, this will be available when you click on configure link in admin/structure/blocks, the url will look like admin/structure/block/manage/{block_id} once you get the block id place it in the below code snippet

$block = \Drupal\block\Entity\Block::load('block_id');
$block_content = \Drupal::entityManager()
return array('#markup' => drupal_render($block_content));

Note: Make sure you replace the 'block_id' with your custom id