I could find two answers from Google on this question.
One is a video by John Mueller - https://youtu.be/uJGDyAN9g-g
The above video highlights the point that subdomain or subdirectory both are fine from Google's point of view. You have to pick one strategy which you can maintain, and makes sense.
And there is another answer from John Mueller in an interview which provides additional insights: https://www.brainlabsdigital.com/blog/john-mueller-interview-at-searchlove/
we try to figure out what belongs to this website, and sometimes that
can include sub-domains, sometimes that doesn’t include sub-domains.
My understanding is that Google can treat the subdomain like an 'independent website' provided the content is different from the main domain. My view is that Google is free to share or not share ranking signals with the main domain based on how it perceives a subdomain.
Note: I didn't mean unrelated content on subdirectory is going to get automatically picked up by Google as related to the core content. Thumb rule (again my view): For closely related content I will prefer subdirectory. For content that is appreciably different, I will prefer subdomain.
UPDATE: Regarding the blog part, I will go for subdirectory provided the blog is closely related to the product/service (central theme) of the main domain.