Custom field not showing in Headline Dynamic Data

Hi,
I am trying to create a directory. As part of doing so I will have various fields displayed on single posts that I would like to then be included in search options. (I was going to use the wp-show-posts/ but now think it should all be possible using GPBlocks)

I have created the custom field (Blockchain:) by simply creating a ' new custom field ' at the bottom of the post as is possible in Wordpress.

The problem is that when I add a headline block and then go to dynamic data it is not showing up, actually I seem to have lots of custom fields and none of them show. All it has as an option is 'ub_ctt_via' which isn't even in the custom field list!?

I have set the Data Source as Current post, Content Source as Post Meta.

Any ideas what I am doing wrong?
Do I have to use a plugin to create the custom field?
Another post suggests that if I use ACF it might work but I didn't want to add a whole plugin just to create a custom field.

Thanks in advance,
Rob