Right; mistakes happen, but it's a matter of the driver's attitude that makes the difference.
I was getting on at Cleveland Circle a few weeks ago and was pulling myself up onto the stairs onto the trolley, which is a tedious effort for me, and the driver yelled at me "come on, come on, I gotta close the door." I wrote to the MBTA about this, and they wrote me a really nice apology and said the driver has been spoken to and would receive sensitivity training.
I really think though that in the case you mentioned with the blind person, the drivers really shouldn't just assume that people can move quickly. This leads to a lot of doors getting slammed on people. It's like how people driving their cars in this town speed up through crosswalks before the person is out of the way, anticipating their speed. If the person missteps or falls or stops, they're going to get run over. It's the same deal with the T doors; they glance in the mirror and see a person and they assume it's a young able-bodied person who will be out of the way by the time the door shuts.
Another assholish green line operator story...I was on the T a couple years ago with a client who very visibly has Down syndrome. He showed his T access pass, and I walked on behind him and said "attendant" and nodded toward him. The driver told me, "No, you gotta pay." I told her that the TAP includes one attendant free of charge, and she said "No, those passes are for THOSE kind of people." Right in front of this gentleman. I called the T and they called back a few days later and said the operator had acknowledged the incident and was fired.