The author of this blog must have uploaded an image without the big red block to mask the private key, realized their error, and replaced it with an image of it blocked. However they didn't remove the unblocked image first. Ghost (the blog engine here) just appends a number to the image when you replace it without changing the filename, so it's easy to find. See the two URLs below:

https://blog.ropnop.com/content/images/2018/05/base64_decode...

https://blog.ropnop.com/content/images/2018/05/base64_decode...

This is probably a good reason to use burner keys for proof of concepts.