The blank template is used for any custom work or when you want to start from scratch on a slide. This is best used for content that doesn't quite "fit" into the other templates.
It is important to keep the styling of your custom slides similar to the rest of your deck.
To create a new slide with a template, click the top left "+ Add" section and then "Create New Slide". From there, you can search and find the template you are looking to use.
Blank Template
Example of a slide that uses the "Blank" Template.