Unless the ISP in question also controls a certificate authority, it is not possible for it to block an individual website served over HTTPS by altering the status code. The fact that you get a 451 indicates that the server is blocking you.
[edit: DNS over HTTPS and other secure-by-default policies in most modern browsers also frustrate ISP blocking/hijacking of individual sites]
I think I just realized what they're doing. Here's what I think is happening: my current ISP just downright blocks the HTTPS site, but when you try to access it through HTTP, you get a blank page with text in bold type that says "HTTP 451 – File unavailable For Legal Reasons". However, other ISPs show you a different page: a blank page all the same, but with a different message ("For reasons beyond [ISP]'s control, this webpage is blocked").
It could be that they are injecting these pages to the visitors and that my current provider just copied and pasted the status error's default text, instead of posting a custom message, like the other company. If it was the page who's blocking me, based on my geolocation, the 451 error should come up both over HTTP and HTTPS, but it doesn't. It's either that, or my ISP is altering the status code only for the HTTP site, and blocking access to the HTTPS one.