It’s usually better to have as clean of a URL as possible. And for that reason, in most cases, I would not add /blog/ to your URLs. If you are really trying to rank your website for the keyword “blog”, then this might be an acceptable practice to add /blog/ to your URLs. Allowing the blog posts to have cleaner URLs without the directory could incentivize users to write more. It could also allow you to have a higher click-through rate on Google.