Whitelist Custom Block For Calling in Static Block with Double Curly Brackets

When a block directive, double curly brackets: {{block type="module/block"}} is called in the static block, it is necessary to whitelist the block type in
admin > System > Permissions > Block. Otherwise, it will not render.

Alternatively, the block can be whitelist in the table permission_block directly.

Reference Mage_Core_Model_Email_Template_Filter::blockDirective()