I have to say i have, in my whole life, never heard of an residential ISP blocking a single port (at least if you don't live in china, russia or similarly run countries). If they have an issue with services you have running, they normally either contact you or disable the connection as a whole first and ask questions later. There are some shenanigans regarding traffic shaping tho (often used to annoy torrent users). Are you on a shared medium (a.k.a fiber to the curb)? In that case, if you generate loads of traffic, they might throttle - but in most cases not block - your connection, because it can cause an already overbooked line to generate issues in your local area; but this would persist even when changing ports.
What i have encountered more often are consumer level routers that don't handle lots of connections at once well, causing them to freeze or drop traffic. My ISP provided cable modem / router (in my case in bridge mode so i can use my own router) freezes for a minute when starting torrents and not limiting the amount of connections created per second, and repeats the freezes every time the connection comes back and the torrent client continues to overload the router.