In order to configure Magento 2 Alternate Hreflang Extension for blog post page, first of all, navigate to Content > Blog > Posts and choose the post you want to configure the alternate hreflang on. Then find the Alternative Store View Localizations section and set the alternative hreflang for the blog post page. In order to do so you have to start entering the page title in an alternative language and choose the corresponding page from the output.

Alternative store view localizations option

Finally, once you've finished don't forget to press the Save button.

Alternate hreflangs for blog post page
Check out this short video to follow the instruction and see the result: