'm looking at your question from two different angles. One is about niche blog. While the other is about a topic for my blog. I wish to give you the two responses in case your question is actually the one I have in mind.
Choosing A Niche Blog:
So, let's start with the niche blog. If you have a blog, it should be about something. But if you haven't decided what your blog should about already, here are a few thoughts that might help.
The first thing to do is ask yourself, what do I want to tell the world? Your blog is your voice, what do you want to say?
You should know deep inside. Don't flow with the general thoughts of what are the best niches to blog? Or which niche is a high revenue earner?
Those kinds of thoughts don't always produce successful blogs. The moment you are after money, you forget that blogging is about giving the world something before you can take back.
If you are not giving your best there may not be anyone to give back to you. There are hundreds of millions of blogs out there, how do you intend to compete if there is nothing unique about your blog.
And if you are looking for that unique thing people would love about your blog, that's you. If you don't know already, I'm letting you know now.
You are the niche your blog is looking for.
Topic For My Blog:
That brings us to the next part of my answer, what topic should you blog? I don't know, honestly. It's like you are asking me, what's on your mind? I can't possibly know what's on your mind.
In fact, that's what your topic should be about. I'm going to give you a candid advice, you can actually blog about any topic you want in this world. Even if you know little about it. The truth is, nobody really knows much about any topic in the world.
So, you are now wondering, how can you compete with those who know more than you? It's by simply telling your story from your own perspective.
If you have followed my response, I sounded off from all the advise everyone else has given you. It was intentional. I wanted you to see that there is nothing like blindness..
It's either you are seeing the light or what you see is only darkness.
So, you can't possibly know what to write. But If I were you, I would take a pen and write the same heading as my question and fill it with my own answers. Because only you can answer this question more than anyone now.
In concluding the topic for my blog, choose any niche you are comfortable with and learn everything you don't know about it, then start sharing your own perspective on that niche. See what I'm writing about on my blog BUSINESS75.COM - Do You Want A Profitable Business Blog? (http://www.business75.com)