As far as I know, you can’t pass a context to Template.contentBlock, 'cause the context that is automatically assigned to it is the one of the current invoker of the template.
Take an example:
<template name="foo">
{{> Template.contentBlock ..}} {{! `..` could be the helpers defined for `something`}}
</template>
Template.foo.helpers({ someFooHelper: =>(){} });
So if I call:
<template name="home">
{{#foo}}
{{someFooHelper}} {{! doesn't work 'cause the context assigned here is the `home` one. }}
{{/foo}}
</home>