Obviously it's wrong to talk to a guide dog instead of the person:
Don't talk to then guide dog instead of the blind person, this is insulting and the dog never answers.
For some reason I was taught that you don't discuss a guide dog with a blind person, as it focuses on their disability and it's better to make small talk about other things. This is when you may encounter a blind person you do not know.
In our neighbourhood we have quite a few blind people and sometimes people will get to know each other's voices and say hi in the street or the shops.
I was thinking, if I see a person with a dog, we'll usually discuss the dog, as they're usually cute and interesting.
I was in a shop and a blind lady didn't have her dog, she was using a cane. She started up a chat, as my son was being cute and she asked how old he was. I then asked where her dog was and instantly I felt bad. (I'd broken this invisible rule - I'm not sure where it came from).
When starting up a chat with a blind person should one avoid discussing their dog?