Support for Block field

Also field support for Views Transforms is definitely on the wishlist.

You can write your own Field Transform plugins and we're currently working on a guide for it.
But a good example for it is to check out some existing ones, such as src/Plugin/Transform/Field/EntityTransform.php

As for field support for Views Transforms, that's our next goal within Views integration.

Thanks, I got it working.

Here's some proof-of-concept code for anyone interested - Block field support for Views and Webform blocks. (Not using content/custom blocks but understanding what's happening here, should be easy to implement.)

namespace Drupal\transform_api_block_field\Plugin\Transform\Field;

use Drupal\Core\Block\BlockManagerInterface;
use Drupal\Core\Field\FieldDefinitionInterface;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\group\Context\GroupRouteContext;
use Drupal\transform_api\FieldTransformBase;
use Drupal\transform_api_views\Transform\ViewTransform;
use Drupal\views\Plugin\Block\ViewsBlock;
use Drupal\webform\Entity\Webform;
use Drupal\webform\Plugin\Block\WebformBlock;
use Drupal\webform\WebformTokenManagerInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Transform field plugin for block field items.
 *
 * @FieldTransform(
 *  id = "block_field_transform",
 *  label = @Translation("Block field transform"),
 *  field_types = {
 *    "block_field"
 *  }
 * )
 */
class BlockFieldTransform extends FieldTransformBase {

  /**
   * The block manager service.
   *
   * @var \Drupal\Core\Block\BlockManagerInterface
   */
  protected $blockManager;

  /**
   * The group content storage service.
   *
   * @var \Drupal\group\Entity\GroupRouteContext
   */
  protected $groupRouteContext;

  /**
   * The token manager service.
   *
   * @var \Drupal\webform\WebformTokenManagerInterface
   */
  protected $tokenManager;

  /**
   * Constructs a BlockFieldTransform object.
   *
   * @param string $plugin_id
   *   The plugin_id for the transform.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
   *   The definition of the field to which the transform is associated.
   * @param array $settings
   *   The transform settings.
   * @param string $label
   *   The transform label display setting.
   * @param string $transform_mode
   *   The transform mode.
   * @param array $third_party_settings
   *   Any third party settings.
   * @param \Drupal\Core\Block\BlockManagerInterface $block_manager
   *   The block manager service.
   * @param \Drupal\group\Entity\GroupRouteContext $group_route_context
   *   The group route context to get the group entity.
   * @param \Drupal\webform\WebformTokenManagerInterface $token_manager
   *   The token manager service.
   */
  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, $label, $transform_mode, array $third_party_settings, BlockManagerInterface $block_manager, GroupRouteContext $group_route_context, WebformTokenManagerInterface $token_manager) {
    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $label, $transform_mode, $third_party_settings);
    $this->blockManager = $block_manager;
    $this->groupRouteContext = $group_route_context;
    $this->tokenManager = $token_manager;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $plugin_id,
      $plugin_definition,
      $configuration['field_definition'],
      $configuration['settings'],
      $configuration['label'],
      $configuration['transform_mode'],
      $configuration['third_party_settings'],
      $container->get('plugin.manager.block'),
      $container->get('group.group_route_context'),
      $container->get('webform.token_manager')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function transformElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    if ($items->isEmpty()) {
      return $elements;
    }

    foreach ($items as $item) {
      // Assuming each $item contains the plugin_id and settings.
      $plugin_id = $item->get('plugin_id')->getValue();
      $settings = $item->get('settings')->getValue();

      // Create block plugin instance.
      $block_plugin = $this->blockManager->createInstance($plugin_id, $settings);

      // We currently only handle Views blocks and Webform blocks.
      // Views blocks leverage the ViewsTransform plugin, while Webform blocks
      // get the form elements and transform them as a simple field list.
      if ($block_plugin instanceof ViewsBlock) {
        $args = [];

        // If you need to pass contextual filter values, add them here.
        // You can get any other fields from the parent by loading the entity
        // with $items->getEntity(); - this is useful if you want to get a
        // category, or some other setting the View needs.
        //
        // Get the View id and display id, this is one way.
        $plugin_id_array = explode(':', $plugin_id);
        $view_name_array = explode('-', $plugin_id_array[1]);

        // Transform API Views submodule provides a ViewsTransform plugin.
        $elements[] = new ViewTransform($view_name_array[0], $view_name_array[1], [], $args);
      }
      elseif ($block_plugin instanceof WebformBlock) {

        // Get the Webform ID from the block settings.
        $webform_id = $settings['webform_id'];
        // Load the webform.
        $webform = Webform::load($webform_id);
        if ($webform) {

          // Return only the form elements.
          $webform_elements = $webform->getElementsInitialized();
          $webform_elements = $this->tokenManager->replace($webform_elements, $webform);
          $elements[] = [
            'type' => 'webform',
            'webform_id' => $webform_id,
            'fields' => $this->processWebformFields($webform_elements),
          ];
        }
      }

    }

    return $elements;
  }

  /**
   * Removes the hash from the keys of the given array for Transform API.
   *
   * Also turns the root level keys to numeric.
   */
  protected function processWebformFields(array $array, $level = 0) {
    $result = [];
    $index = 0;

    foreach ($array as $key => $value) {
      // Remove the leading '#' from the key.
      $newKey = ltrim($key, '#');

      // If the value is an array, process it recursively.
      if (is_array($value)) {
        $value = $this->processWebformFields($value, $level + 1);
      }

      // Add the new key-value pair to the result array.
      if ($level === 0) {
        // For the first level, use numeric keys.
        $result[$index] = $value;
        $index++;
      }
      else {
        // For deeper levels, retain associative keys.
        $result[$newKey] = $value;
      }
    }

    return $result;
  }

}