Depending on your point of view, the first good idea you have when starting a Blog can be either a catalyst or a disaster. If you ascribe to Tom Monahan's viewpoint, that first inspiration is never a wise one to pursue. There is some merit in consideration when it comes to deciding what it is you are going to Blog about, but this form of writing is a long way from the carefully structured world that other types of writers live in. Blogging, like Twitter, is becoming less and less about what is said and (in many cases) revolves around quickly you can say it before anyone else does.
When faced with such instantaneous surroundings, blogging could end up losing a lot of impact if you are the type of person who needs to think before they post. However, it needn't be the case that being slow is a disadvantage. Even prolific bloggers run out of creative steam sometimes, and there comes a point where, if you feel you're having to invent reasons for posting, it could be that it's just time to take a moment. It doesn't have to be about making your mark on a constant basis. This is about your needs before anyone else's.
When there are no staff to employ, no guidelines to follow and no revenue to secure, blogging is at its best: I am not a fan of those who feel the need to 'benefit' financially from their efforts in this regard, and you will never see me succumb to the lure of ads. No, really, you won't, even if I become the most famous blogger on the Planet overnight, this site remains free. My first idea, when I started this blog, wasn't to one day make my fortune from this. I write here because I want to talk to other people about a game I love, not to make money on the back of it. Maybe one day I might get lucky with a piece of my fiction. Who know, perhaps someone will one day employ me to write this kind of drivel on their site. That's cash I'd happily take, but not here. Sorry to disappoint you, but principle is fairly important to me.
That's the key to your first idea: being true to it. If it's not something you feel you can write about even when you hate your subject matter in a day of darkness then maybe you shouldn't be writing at all. Perhaps your idea will then make you realise that actually, you enjoy talking about something else instead which you only realised when you'd taken the first step. Feel free to change your mind. No-one is going to have a problem if you do, but remember that if you want to keep your audience, you're going to need to explain to them what's going on. I think some of the best blogs I read currently are those written by people who started in one place and are constantly moving to different places, re-assessing their needs and priorities, whilst simultaneously remaining true to their need to write.
This is the indicator of a person who feels comfortable in their writing skin, and always makes for the best reading, at least for me.
The first idea for your blog, whatever it is, will not matter in the end anyway. What will be far more significant is the fact you chose it, and it is one of the ways that readers will come to identify your own voice. Don't worry if you get it wrong. Just remember that the Internet remembers everything, and when you look back on those early posts in a decade's time, just how much you have grown in the intervening period.
That's the fun in the journey.