Magento 2: Call a static block using xml

<block class="Magento\Cms\Block\Block" name="Promo">
    <arguments>
        <argument name="block_id" xsi:type="string">promo</argument>
    </arguments>
</block>

This is equivalent to M1.

Try below code.

Call from Phtml File:

<?php echo $block->getLayout()->createBlock('Magento\Cms\Block\Block')->setBlockId('block_identifier')->toHtml();?>

Call from cms page or block:

{{block class="Magento\\Cms\\Block\\Block" block_id="block_identifier"}}

Call from Xml File:

<referenceContainer name="content">
    <block class="Magento\Cms\Block\Block" name="block_identifier">
        <arguments>
            <argument name="block_id" xsi:type="string">block_identifier</argument>
        </arguments>
    </block>
</referenceContainer>

Hope this helps you!

Call from Xml File worked. Change the "block_identifier" from the CMS block created on magento dashboard and add it to layout.xml

<referenceContainer name="footer">
       <block class="Magento\Cms\Block\Block" name="block_identifier">
            <arguments>
                <argument name="block_id" xsi:type="string">block_identifier</argument>
            </arguments>
        </block>
    </referenceContainer>