I think there are two questions. Can a Blair student not in the magnet take magnet classes? Yes for almost everything as long as they have the prereqs. I think 9th/10th science/cs/research are closed but upper level classes are open.
Can an MCPS student not at Blair take magnet classes? Pre-covid we heard they made an exception for one student, but haven't heard of anyone recently. It may still be possible on a case-by-case basis. Reach out to the magnet coordinator and ask.

NP. How do you explain on college applications that you weren’t in the magnet program for a reason other than “didn’t get in”?

Students who go to Blair that are not in the SMCS magnet program can take the magnet classes in 11th and 12th.

Students not at Blair can not take any classes at Blair.

My son is at WJ and would like to take courses beyond the AP offerings there. He is already doing DE at Montgomery College, but there are courses in the Blair magnet program he'd like to try. We didn't apply for the magnet program because we moved here too late to apply.