Magento 2 How to create CMS block with HTML content programmatically

Below code work for me.

Create InstallData.php file at /app/code/Custom/Module/Setup/InstallData.php

<?php

namespace Custom\Module\Setup;

use Magento\Cms\Model\BlockFactory;
use Magento\Framework\Setup\InstallDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;


class InstallData implements InstallDataInterface
{
    private $blockFactory;

    public function __construct(BlockFactory $blockFactory)
    {
        $this->blockFactory = $blockFactory;
    }

    public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
        $cmsBlockData = [
            'title' => 'Custom CMS Block',
            'identifier' => 'custom_cms_block',
            'content' => "<h1>Your HTML content added here</h1>",
            'is_active' => 1,
            'stores' => [0],
            'sort_order' => 0
        ];

        $this->blockFactory->create()->setData($cmsBlockData)->save();
    }
}

If your module has already been installed then use UpgradeData to create CMS block programmatically. Now, execute this below command.

php bin/magento s:up
php bin/magento s:s:d -f or php bin/magento s:s:d
php bin/magento c:c

Below code works for me.

<?php

    namespace Custom\Module\Setup;

    use Magento\Cms\Model\BlockFactory;
    use Magento\Framework\Setup\InstallDataInterface;
    use Magento\Framework\Setup\ModuleContextInterface;
    use Magento\Framework\Setup\ModuleDataSetupInterface;
    use Magento\Cms\Model\BlockRepository;


 class InstallData implements InstallDataInterface
{
    private $blockFactory;

    public function __construct(BlockFactory $blockFactory)
    {
        $this->blockFactory = $blockFactory;
    }

    public function install(ModuleDataSetupInterface $setup, 
        ModuleContextInterface $context)
    {
        $cmsBlockData = [
             'title' => 'Custom CMS Block',
             'identifier' => 'custom_cms_block',
             'content' => "<h1>Your HTML content added here</h1>",
             'is_active' => 1,
             'stores' => [0],
            'sort_order' => 0
        ];

    $block = $this->blockFactory->create()->setData($cmsBlockData);
    $this->blockRepository->save($block);
    }
}

the use of the save() method is deprecated if called directly from BlockFactory, the use of BlockRepository is recommended.