Blog custom filter problem

I'm having new difficulty with perch_blog_custom filtering.

It seems I can only 'filter' by "native" blog content IDs. I cannot seem to filter by blog content IDs that I have entered into post.html.

perch_blog_custom(array(
    'filter' => 'sectionID',
    'match' => 'eq',
    'value' => 2,
    'template' => 'post_in_list.html',
));

^ This works as expected.

perch_blog_custom(array(
    'filter' => 'myCustomInput',
    'match' => 'eq',
    'value' => 'foobar',
    'template' => 'post_in_list.html',
));

^ this does not work.

Any shared experience with this?

Additionally, when I look into <perch:showall /> I see that any input I added gets another field with a perch_ prefix. Such as myCustomInput and perch_myCustomInput

I tried using this perch_myCustomInput in the filter to no avail.

Many thanks, Nick