How can I become expert enough to blog about a topic ?
Well lets be honest here.. an expert at its core is just someone that knows more than you do and that certainly varies in degrees as well. There are people on any number of topics that know a little more than you and there are others that know a lot more. In the realm of blogging, you don't start a blog because you're an expert. You start a blog because you want to write about something that you know about, that you're passionate about, that you love and want to share with others.
Nevertheless, if you can't buy into what I've said so far, then take this avenue instead... Start to follow other experts and learn all that you can about the topics that are associated with your niche. Soak it up and make connections with the people that are most respected in your niche. Get involved in some joint ventures with them which will help you to increase your exposure. This will also help to expand and improve the way you are perceived. When you are looked upon as someone very knowledgeable then the title of expert may be something that people start to associate with you. Additionally, when you start to teach others what they want to know within the niche that you're in this is yet another step that can be extremely helpful in being seen as an expert too.