As far as I know, it doesn't matter much from a ranking perspective but google does find it easier to categorize for search results if you use /blog.
In most cases /blog is a lot easier and flexible to setup and manage.
You could also forward your root (example.com) to example.com/blog if you don't need one.
i will prefer mysite.com/post.html, as we have done for our agency website..https://bossseosanfrancisco.co/blog/
I dont think it matters a huge amount. Many people do the latter these days so there might be some preference there. But to be fair sometimes you see 2 subfolder before the post url title which can help if you have a large volume of posts across topics.
Personally I prefer the former (with single sub-folder) as 1) rather than 'blog' I'll use words that suit overall site article SEO purposes and 2) it works nicely with analytics to grab all posts in one sub-folder for build analysis.
Overall I dont think it has a huge impact.