Exactly so. If you need special conditions, then you need to make a reservation. If there isn't one to be had on the train you'd ideally like to use, then surely you have to modify your travel plans. You can't expect just to turn up and have space created for you.
I really hate this attitude, which is common among public transport companies too.
IMO, equality means provision for as close as possible the level of service that non-disabled people get. Non-disabled people can travel on the train without having to make plans in advance. Disabled people should be able to do the same. And if it costs extra to make that provision, then the service provider should stump the bill, because that's what it takes to provide a service that's available to all.
Sometimes it's not possible to provide the same level of service. (For example, wheelchair users at unstaffed stations without lifts.) But this is not one of those times. A blind person is less capable of standing on a moving train than a sighted person. So, if no passenger was willing to give up their seat, then the conductor should have turfed someone out of theirs. That might make someone's day a bit worse (having to stand), but if you compare that mild inconvenience to how much it would suck to be blind, they should hopefully get some perspective, count their blessings, and do the thing that helps out a fellow human.
The issue of reserved seating is a red herring. I've never encountered a train where every single seat had a reservation on it — but even if that was so, I still think access needs should trump reservations. After all, if a train is short formed and you have a reservation in the missing coach, this forum is quick to point out that a reservation isn't a guarantee, and since you didn't pay for it you're not due recompense. The same should apply here.