My mother was a vet tech and pretty much passed her knowledge to me, it wouldn't be cancer of any severity if she isn't showing discomfort OR stress. Urinating in the house could mean anything, the litter box is hard for her to get to OR another cat was in the house. Don't worry about the fact she's blind ( no effect on her health unless she gets a concussion ) either a dog urinated or another cat was in the house. I wouldn't put her down if she enjoys life. This is just my opinion.
Fact: Once a cat urinates in one spot, it will continue to use that spot. Cleaning will not work, the urine has high levels of ammonia and can not be eliminated unless that part of the carpet is replaced.