So, after much back and forth with my ISP (including a factory reset and replacement of my modem) and after subsequently opening a ticket with Backblaze, it seems that someone, somewhere, has incorrectly flagged some address(es) at their East Coast datacenter as malicious, which has resulted in them being blocked:
We have been made aware of a subset of customers receiving connection issues with our East Coast datacenter. We believe that internet monitoring tools inaccurately marked our subdomains f005.backblazeb2.com as malicious websites. Our compliance team is working on correcting this false reputation.
We do recommend reaching out to your ISP as well to report the false positive. We’ve found that these reports tend to be taken more seriously when it comes from direct customers.
At this point, I have contacted my ISP as Backblaze recommended, and I suppose it will be a matter of waiting until the complaint trickles through their bureaucracy to reach the person who can actually do something about it.