I write on a specific but growing "political" movement. I deal with personal development specifically for them. It's a pretty tight niche, but there is virtually no competition and as it grows around the world I'll grow with it!
I find any topic that is controversial is always a huge success. You get it shared by people that agree and want to show it or to annoy other people. Then, you also get it shared by people that hate you because they want their friends to see how edgy they are. It's a win-win as far as numbers games go.
I tried to stick strictly to one part of personal development for the target audience, but then I got bored and opened it up to things I thought would interest them. Opening up more of these categories was a huge success and actually saved me from ditching the project. So if you pick something, don't be afraid to test different things out.
If you want to make money, anything to do with money is always a good bet. My financial articles usually bring in the most dollar signs. But that is fairly obvious. More people will pay you to save them double the money later than they will pay to hear you talk about your problems with tooth picks.
Don't pick a stupid blog topic that has already been done 500000000 times unless you can lead some serious revolution to the current ones.
Good luck!