That depends on whether the blog subdomain is in the main site's GA referral exclusion list.
If it's not, then the blog should be in the referrals report and that should answer your question.
However, that would be considered a self-referral and it's a good practice to fix it. More details on it here: https://support.google.com/analytics/answer/6350128?hl=en#zippy=%2Ccross-subdomain-sessions-and-the-referral-exclusion-list
Now, how do we answer the question if there's no self-referrals? With custom reports. A bit more complex, but a bit more proper, too. Here, I made an example for facebook. you just use your subdomain there:
Here's the result:
This should answer your referral traffic quesion. Just note that this will highlight sessions where at least one hit had the referral. It doesn't have to be the first hit, could be in the middle of the session. Well, same applies to the above mehod.
To find sessions that started with blog, it may be easier to implement explicit tracking for that via GTM.