Hallo YorkshireLass,
My mum is also blind. She was also very frightened and distressed and believed that a neighbour coming in every night to blind her, because each morning when she woke up she thought she had lost her sight the night before. It was perpetual terror for her and anguish for me to find that there was nothing I could do or say that reassured her. I totally agree with your feeling that other terminal illnesses are given medication to improve their quality of life, even though it may shorten it. I had to really press the mental health team for some medication and in the end said to the consultant that I fully understood that anti-psychotics could have serious or even fatal side effects but that given mum's state, that was a risk I was willing to take. Mum was given risperidone, had no side effects at all, and is now content and no longer delusional. It doesn't work like that for everyone - she was lucky that it worked so well for her - but I strongly believe that in our case it was a risk worth taking, given that her quality of life before was non-existent. If you want medication for your mum then keep pressing for it, and let them see that you understand that there are some risks.
On the general point of memory tests for the blind, the dementia nurse tried to give mum a modified test but was not very successful. I thought I could have presented some of the tests to mum in a way that better showed her ability. But in the end they just said "probable mixed dementia" and have just left it at that. No score, no scan or anything else. Mum is very elderly so perhaps they thought it was not worth putting in the resource. I can't say the lack of a "proper" diagnosis has ever bothered me. Mum has problems and I have to deal with them. Giving it a label doesn't make the problems go away, though it is easier to explain to others by just saying mum has dementia.