Is it better to have a general blog or should it be related to a specific topic?

Having a blog which is "General", as you say, will not be of much benefit to you. Yes there are sites/blogs that are producing insane amount of content on vairous topics but the problem is they are no expert.  When people need to solve a particular problem they will go blogs that will save them time and get them an answer from an expert. 

When you have a blog focussin on particular area it helps. People will see your blog as the go to option when they need an expert opinion about problems related to that particular domain. By focussing on specific area of writing you also get the chance to showcase your expertise and in turn enhance your professional credibility.