As the related question may come up, be aware of what your local government's laws are related to disabled persons. For example, the American's with Disabilities Act is:
The ADA is one of America's most comprehensive pieces of civil rights legislation that prohibits discrimination and guarantees that people with disabilities have the same opportunities as everyone else to participate in the mainstream of American life -- to enjoy employment opportunities, to purchase goods and services, and to participate in State and local government programs and services.
Your laws may vary, but the ADA does not apply to user testing for accessibility in many respects. Yes, it is just as important to know what doesn't apply as what does apply. There are some aspects (such as providing a safe environment for the disabled individual) that would still apply to you case, but you'd want that anyway!
This leads into your question about...
How should I ask this question in a non-offensive and non-derogatory way: "What is your degree of blindness?", "Are you legally blind?...".
... because it is illegal for a business owner to ask these sorts of questions to someone. In your case the person who stands before you is a volunteer (or mildly paid subject) who is well aware of the reasons they are there.
"Are you legally blind?" - perfectly valid, straight forward, and mature question that can be answered easily. You are recruiting legally blind people, so to ask the question is not insensitive.
"What is your degree of blindness?" - eh... that's relative. Always ask in legally or medically valid terms when referring to the disability. This (1) protects who from being a "jerk" (or worse), (2) is a language the participant will understand, and (3) does not leave the question open to interpretation or exaggeration.
Next, I want to ask about their experience with digital media, and ask them if they have any idea how different their experience is comparing to a normal user.
As you point out, use words like "normal" very carefully! This would also not be the best question because it is open to such a wide range of interpretation.
What if this person only knows other blind people, or sighted people who are just really bad at using computers? What if this person lives in a neighborhood full of computer scientists?
You should ideally test both groups yourself - legally and non-legally blind individuals. Then make that comparison yourself... through the power of statistics!
Here are a few sites with additional information that also hit on a few of your other questions:
Accessibility in User-Centered Design: Conducting Usability Testing
Regarding setting up the testing room, they suggest:
- Don't move anything without asking first when at the participant's
- Record screen reader audio output.
- Watch the keyboard.
Regarding getting feedback, it suggests:
Provide documents in the participant's preferred format, as requested during participant recruiting. Some people read braille very quickly, and some read it very slowly. To help manage time, you may want to ask participants if you can read the documents aloud and provide the braille version for reference. Be prepared for some people to want to use the braille themselves and not have you read it.
The short version to all these, is be prepared to offer alternatives. Don't assume.
The American Foundation for the Blind has an article hitting on exactly your situation: Conducting Usability Research With Computer Users Who Are Blind or Visually Impaired
The paper should be read in full, so I will simply quote the abstract:
There are three main points to this paper:
To illustrate the frustrations of accessible web sites. That is, even technically compliant sites can be inaccessible to the user, because they are so difficult to use. Compliance with accessibility laws and guidelines, in other words, is necessary but not sufficient for users to access what they need. I hope that illustrating this highlights the importance of usability studies as integral to web design.
To give some background and information about how to conduct usability and user experience studies. I am particularly interested in how they can be adapted for and what may be unique about computer users who are blind or visually impaired. I will include a discussion of the added value that the web offers to this community.
To present generalizable findings, the results of research conducted by the American Foundation for the Blind (AFB), which we hope can be applied more broadly.