This is a class name
div class="blocks-promo" and not a block, block in Magento mean other thing eg:
vendor/magento/module-catalog/Block/Product/Price.php is a Magento block.
I guess you have found this class in some template and you want to get the related block of that template right ? if it is, a simple way to do is to
enable the template path hints AND
Block name hints then you will find what you need.
Store > Configuration > Advanced > Developer > Debug > Enabled
Template Path Hints for Storefront > Yes
Store > Configuration > Advanced > Developer > Debug > Add Block Names
to Hints > Yes
Note: Don't forget to change storeview scope on the top left corner.
Right: the block type.
Left: the phtml path.