This is an example script on how to export content from the Sanity blog schema out to markdown.

sanity-blog-content-to-markdown.js

/**
 * This is an example script on how to export content from
 * the blog schemas out to markdown.
 *
 * Run this script in a Sanity project folder with
 * $ sanity exec markdown.js > blogposts.md
 * on a public dataset, and
 * $ sanity exec --with-user-token markdown.js > blogposts.md
 * on a private dataset
 */
import client from 'part:@sanity/base/client'
import toMarkdown from '@sanity/block-content-to-markdown'
import imageUrlBuilder from '@sanity/image-url'
/**
 * This function can build an image from
 * an asset reference with hotspot and crop
 * */
const urlFor = source => imageUrlBuilder(client).image(source)

/**
 * We use the project config to resolve images in
 * block content
 */
const { projectId, dataset } = client.clientConfig

const { log } = console
const serializers = {/* Custom blocks here */}

function getData(query) {
  return client.fetch(query)
}

async function parseToMarkdown(data) {
  data.forEach(({
    title = '',
    _createdAt,
    _id,
    _rev,
    _type,
    _updatedAt,
    author = undefined,
    body = [],
    categories = [],
    mainImage,
    slug
  }) => {
    /** Output title */
    log(`# ${title}`)

    /** Byline */
    log(`First published: ${new Date(_createdAt).toDateString()} – Updated: ${new Date(_updatedAt).toDateString()}`)
    if(author && author.image) log(`![${author.name}](${urlFor(author.image)})`)
    if(author && author.name) log(`by ${author.name}`)
    log(`\n`)
    /** Main blogpost image */
    if (mainImage) log(`![](${urlFor(mainImage).url()})`)
    /** Rich text with no custom block types */
    if (body) log(toMarkdown(body, { projectId, dataset, serializers }))
    if (categories) log(`Filed under: ${categories.map(({title}) => title).join(', ')}`)
  })
}

(async() => {
  const query = `
    *[_type == "post"]{
      ...,
      author->,
      categories[]->
    }
  `
  const data = await getData(query).catch(console.error)
  await parseToMarkdown(data)
})()