Customise category tag on image in Flex Block

Hi – loving the Newspaper theme but wondering: Can I add a tag to an image in a Flex Block based on a custom value from a post?
What I currently have in the Flex Block is an image with a category tag overlaid on the image with the corresponding category of the post. So the article belongs to the category XYZ and the image has a tag overlaid on it that says XYZ.
What I want to do is replace that XYZ tag with a custom value from the post (in this case, it’s a custom value called ‘source’).
Would appreciate any help you can give.
Thanks!