I have been a blogger long enough to understand that it is better to write about things that you really know about. There is no point writing about things you have a vague idea about. Your readers are going to realize that you have no idea what you are writing about. You need to be an expert in something and sound like an expert if you want to impress your readers. Do not try to impress your readers by pretending that you know about something that you actually do specialize in. Your risk embarrassing yourself and looking silly. You might discover that some of your readers noticed that lack of knowledge about the subject you tried to write about. Here are some thins you should keep in mind:

-Be especially careful about writing on complicated topics like medicine or technology. You may easily embarrass yourself if you make some mistakes.
-If you do not feel like an expert about some particular topic like, for example, some kind of rare disease, then make sure that you mention it at the end of your post. Feel free to write about it, but when you give actual advice, then make sure that your readers understand that they may want to additionally consult some expert on the topic.
-You may easily avoid embarrassing yourself by writing about topics that you feel comfortable about. This way you will gradually gain the trust of your readers and they will know that they can rely on whatever information you provide in your posts.