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

Alternate store view localizations section

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

alternate hreflangs blog category page configuration