Will one flawed grade outside of my major from my freshman year affect graduate school?
It is extremely unlikely that a graduate admissions committee/potential supervisor would look at your first year courses in any amount of detail. They are going to focus on the higher-level courses you take in later years, whether or not you do an honours degree/undergrad thesis, etc.

The only way I can see this affecting you is by lowering the overall average grade for your degree (measured in many places as a GPA). Many departments have a minimum GPA/grade requirement to be considered for admission to grad school. However, as long as you get consistently good grades for the rest of your degree, this little slip-up isn't going to matter much.