On the question of whether to focus a blog or not - I have 3 blogs.
One is the one in my signature but to be honest I've not really put a huge amount of effort in to building it yet.
My main one which has been going for around 18 months relates to a particular niche and I mainly make posts about that niche area on it. Just now and then I will post something 'off topic' - labelled as such! I know from emails received etc that people often find this blog useful (which is it's purpose!) and some people will spend a long time just nosing around on it when they first come across it
Oldbag of Cairo for the inquiring minds that want to know!
That said, and even though I have AdSense, I have made not 1 cent out of it - I don't know if anyone has ever clicked an Ad on there! Doesn't really bother me though as it wasn't aimed at money. I have wondered about doing donation buttons for relevant charities on there.
My third blog is a secret
That's because I suddenly had a burning desire to have somewhere I could blog about absolutely anything in the world that took my fancy. I'm partly using it as an experiment to see how long it takes for an 's-curve' to build up!