How to create Blog Post Specific Widget

Since it sounds like you’re trying to do this without code I would do it this way: Create an ‘Advertisement’ post type which supports categories. Then use a ‘Related Posts’ widget that allows you to select custom post types as well as related categories.

Possibly one of these And use the widget settings to call the appropriate Ad

UPDATE: Not sure how clear my answer is but to use your example – you have a blog post in the category ‘dairy’ in your sidebar you use the ‘Related Posts’ widget to display posts of type ‘Advertisement’ in the category of the current post. Obviously, set up your Advertisement posts in the appropriate categories.