Many companies have a company blog. The question often arises, which is better for SEO? A subdomain or a subdirectory?
A subdomain can be useful to separate out content that is completely different. Google uses subdomains for distinct products such news.google.com and classroom.google.com for example. Craigslist also uses subdomains. See screenshot image below on how Craigslist’s subdomains appear in the Google Search Results Page:
Google doesn’t treat subdomains as separate domains anymore. It really is more of a branding play, or for different content. I’ve always had a preference to use subdirectories whenever possible. My reasoning was that it was considered by Google to be part of the main domain, and all inbound links & social equity acquired contributed to increasing the algorithmic authority of the main domain. As opposed to subdomains, the links and social equity that subdomains acquire, don’t get attributed to the main domain.
But that’s not the case anymore. Google “may” consider subdomains the same as the main domain. All links and social equity “may” get attributed to the main domain.
When Rand Fishkin of Moz moved a subdomain to the main domain, he saw keyword rankings rise dramatically across the board for every keyword he tracked.
However, there is an advantage of subdomains being treated as separate domains. If you spend enough time & effort in building the subdomain, and everything aligns properly, you can get multiple listings and/or indented listings in the SERPs, as in the screenshot image above. That could be exactly what your Branding objectives are. But, if done wrong, you can have two weak domains and rank for less.
To answer the original question regarding a company blog, I still prefer to use a subdirectory.