Creating a block requires a few files which are linked, the task is very similar to create-page or create-component.
I propose the following command:

mgnl create-block <blockName> [-p <path>]

<blockName>:
The name of the new block.

-p <path>
The path to the light module to add the new block to.
If the parameter is omitted, the command must be run within an existing light module folder.

The command should create these files:

• <light-module>/blocks/<blockName>.yaml
• <light-module>/templates/blocks/<blockName>.yaml
• <light-module>/templates/blocks/<blockName>.ftl

The defaults (prototypes) of the block definition (fields!) and the template script to render them needs discussion.