There are many types of blogs on the Internet. There are also many topics to write about. Some bloggers write about a variety of different topics, while others decide that they want to be portrayed as experts in only one field. I think that each decision has its advantages and drawbacks. One of the advantages of writing about only one topic is that it is a lot easier to be good at it. Eventually, you might be even recognized as an expert in your field if you will be really good at what you do. That can be especially the case if you put a lot of work into writing articles. On the other hand, writing about only one topic might get a bit boring after a while. This might be true if it is a niche in which there are not many interesting things happening. If that is the case, then you will be forced to write about some other topics anyway.
There is no clear rule what to do, and nobody is going to make you write about any particular topic. That is why you should remember to choose a niche that is of interest to you so that you would be motivated to write posts. There is no bigger mistake a blogger can make than choosing a topic of his blog that he doesn�t even like.