Adding a JotForm form to a blog post on Wordpress.com is now as easy as placing [embed] tags around your form's URL within your post. Be sure to be on the Text input mode when adding the embed tag.
If your intention is to only have a form on the page, simply copying and pasting the URL on the post body is also enough. Wordpress.com will turn it into an embedded form.
This was a feature that has been asked by hundreds of JotForm users over the years.