How to add blocks to 'contact us' page?

I have added 'contact us' pages with a form to my storefronts. I see that it should be possible to add blocks to the page, because there is a 'blocks' tab. But in the block tab I can not change anything.

I would like to add a block with text for our contact details and with Google maps to show our retail locations.

How do I do this?

If your 'Contact Us' page is a page created in Website>Content then it will use the default Pages location.

Your only option this way is to create your custom HTML block with the Google map as its content in Design>Blocks>Pages, disable it in the Blocks page, then edit your content page, go to the blocks tab, and enable the new custom block for that page only.