Will using the HubSpot blog on a subdomain affect my SEO?

A common concern for new HubSpot clients, and how to respond to it

Moving blogs to a subdomain is a common concern, and one we've raised ourselves too. 
It seems no one is able to agree on whether subdomains are ranked negatively versus your main domain and, if they are, by how much. The consensus among the HubSpot community is that the known benefits outweigh the possible, and mostly unknown, drawbacks. 
We do have it on good authority from an SEO expert that any impact is negligible and, if you do want to mitigate it, best practice is to have a single subdomain for your blog, landing pages, and emails. For example:
  • info.yourname.co.nz/blog
  • info.yourname.co.nz/blog/article-name
  • info.yourname.co.nz/landing-page-name
  • info.yourname.co.nz/view-this-email-in-a-browser

(This is how we'd set this up for you, though 'info'. can be whatever word you want it to be of course!)

This and obviously setting up all the 301 redirects are key to losing as little SEO juice as possible.
Here's a good resource on the pros and cons, and an excerpt below. The most relevant bit to answer your question is here: https://webmasters.googleblog.com/2011/08/reorganizing-internal-vs-external.html. I'd tend to believe Google!

Myth: Using Subdomains is Bad

HubSpot blogs are hosted on subdomains (blog.example.com), which can be a concern for some marketers. But in 2011, Google reorganised its interpretation of internal and external links and announced that subdomains (previously categorised as external links, and which weren’t providing any SEO benefits to your website) would be categorised as internal links. Today, this means believing that using a subdomain for your blog is bad for SEO is nothing more than SEO myth. Google is smart enough to know both www.strategic-ic.co.uk and blog.strategic-ic.co.uk are tied to the same site.