If you have a deeper knowledge about Wix Blocks, you will know that while you can simply drag and drop your custom built Blocks app anywhere on the site, that is not the case with Wix business solutions, such as Wix Stores, Wix Bookings, Wix Blogs etc;
These are Wix built apps, and if you wish to integrate your blocks app in any of these, they have to be added as Plugins, inside of slots, which are specific, pre-defined spaces / locations where you can add your Blocks app.
So if you want to add your custom built Blocks app to your blog post page, you can simply do that by going to the post page, click on + Add Elements > App Widgets and simply drag and drop your widget onto the page.
But if you want to integrate your app directly inside of the Wix Blog / Post widget, there is currently no slot available for the same, as mentioned in my post earlier.