To add Alt text to your image in this case, you would need to build it into your layout code. So in your Blog post layout (list and/or detail) you might have something like this:
The problem with this solution is that there is no copy that actually explains the image with this solution. Technically it adds an alt-tag but it doesn’t really increase accessibility as it doesn’t actually explain the image. If I for example would have a blog post named “Sign up for our talk about space” and use this picture of the moon as a featured image, using the blog post name doesn’t explain the picture. The alt text should probably be something along the lines of “Our moon in the first quarter phase”.
To create good, accessible websites we therefore need support to also add alt text to featured images for blog posts.
