Can a blind person do a post-doc in the humanities?

In the United States there's what is known as the Americans with Disabilities Act (ADA)

which prohibits discrimination based on disabilities and mandates reasonable accomodations. Full or partial blindness will definitely fall under that category, and so they are legally prohibited from discriminating against you and if hired will work with you to help out. Many other locations have similar laws. In Canada the laws vary from province to province, but most of Canada has similar laws. Many (although not all) European countries have similar laws or regulations as well.

Can my blindness be a factor in acceptance / rejection of my application?

As others have said, most Western countries have laws preventing discriminating against you due to blindness. However, the standard is that they you must be able to do the work given a "reasonable accommodation" -- i.e., they could reasonably prevent you from flying a plane or being a sharpshooter, as there is no reasonable way they could enable you to do the job. Since you did your PhD research already, it seems likely that you can do the work despite your handicap.

Note, however, that most post-docs are filled (at least in my experience) by interviewing with a professor or small group of professors. Most professors want to do the right thing; however, it is difficult to do an apples-to-apples comparison between two candidates at the best of times.

At any rate, all you can do is apply and hope for the best.

Are there any helpful websites that I can frequently check for available post doctoral opportunities?

I don't know (and "shopping questions" are actually not allowed here).

Is it necessary that my research should be on a population of a country where I’m studying for my post doc? i.e., if I’m studying in New Zealand, and I want to collect data from people in Pakistan, is this discouraged?

I don't know your field, but I seriously doubt this would be a problem. Here in the US, at least, there is plenty of research into different regions of the world.

What should be my first step for finding a post doctoral opportunity?

I would start by talking to your advisor, trusted old professors, or friends to find out where such jobs are typically announced. They may also be willing to introduce you to their network.

If the goal is a faculty position, I would consider reading Karen Kelsky's book -- plenty of advice there about best practices for preparing for and interviewing for a faculty position; though the book is widely written, she is an anthropologist and has lots of advice that seems like it would apply well to your situation.