I had a negative experience with my undergrad institution during a period of poor mental health, and those health issues are ongoing. Despite efforts to normalize mental illness, college campuses also have a long way to go. I wouldn't recommend this necessarily for anyone in a similar position, but in my applications to grad schools this year I wrote about how my illness has affected my studies and my secondary career goals - namely, that it encourages me to respect the mental/physical health of my peers/coworkers/etc. and be an advocate for the ethical treatment of mental health issues in the academic environment. It was important to me to be up front about my health, and will be important (if I get in... no acceptances yet) that my advisor understands my illness will occasionally affect how I can approach graduate study. This gamble didn't sink me; I've been invited for a visit to one of my top choices.
I'm sorry to hear about your experience with med school - like you said, the depression and burnout rates among med students are terrifying so you'd think they might take a different view on this subject - but if your mental health is managed now, there shouldn't be any rational justification to reject you based on that outright, even if someone does manage to dig up that blog post again. It might qualify as discrimination.
More directly to your question, I've heard about a few high profile cases for undergrad admissions offers being rescinded because of content posted on social media (drugs or highly offensive material). It can and does happen, but I guess from your description of the content, I would be incredibly surprised if it would even come up as a factor in a decision.
But I'm not on an admissions committee so I can't say I have any authority to back that statement up. Best of luck to you.