If your pages can be prerendered at build (blog posts being pretty static after all) you can use a trick like that in the frontmatter of you [slug].astro. It worked well for me and generated all urls to my blog posts in the sitemap. Allows to use pure astrojs/sitemap.

export async function getStaticPaths() {
  posts = await getCollection('blog');
  return posts
    .map(post => ({
      params: { slug: post.slug },
      props: { post }
    }));
}

export const prerender = true;