For our website we want to start a blog in the subdir domain.com/blog but for SEO value we would like the blogposts as domain.com/blogpost1.html . Is this possible in wordpress?
It is possible to install WordPress in its own folder as describe above. If you want a blog post to end with .html that is also possible.
The following link provides an answer on how to do this via code: Add .html (dot HTML) extension to custom post types
Additionally, if you search the WordPress plugin repository, you'll find plugins that do this as well.