eZFlow block: Is there a way to adda checkbox for each block of an ezflow page?

This is possible but undocumented I'm afraid.

Example configuration is following:

[BlockTypeName]
CustomAttributes[]=my_checkbox
CustomAttributeTypes[my_checkbox]=checkbox

Supported values for CustomAttributeTypes are following: text, checkbox, string. If not defined then string is default value.

For checkbox type GUI will render regular checkbox button with values 0 or 1.

Then in templates you can use it like

{if $block.custom_attributes.my_checkbox|eq('1')}
checked!
{else}
unchecked!
{/if}

Hope this helps!