You can not get your HubSpot blog to appear at the subfolder of your root domain unless your main website is also on HubSpot.
If your main website is on HubSpot, you can configure it's domain settings to publish the blog to the main domain - otherwise, you will have to pick a subdomain.
Rest assured, however, the positive benefits of having an active blog with good SEO at a subdomain will outweigh any negative impact of not having it on the root domain.