From the way you worded your question, it sound almost like you're viewing specialization as which historical box you want to lock yourself into. Historians are a flexible people. What you specialize in during the last couple of years in your bachelors may not be exactly what you do a masters in, which may not be what you do a PhD in. I was lucky in that my bachelors uni had a lot of great courses in one the (many) general area of history I was interested in. That worked out well for me.

The skills you develop during your bachelors will be just as important as the specialized knowledge you get in your area of history. My bachelors specialization was in the age of revolution and English/Irish national history. My masters is in conflict studies, and I know of maybe one other student in my cohort who specialized in the same thing as I did. A big part of a master's degree isn't just what you learn, it's the tools it gives you to do the kind of history you want to do. You can, more or less, do whatever you want with those tools.

The other comment is great, I just figured I would chime in as well. Just a note: I’m coming at this from a U.S.-based perspective in academia, so this might be different if you’re somewhere other than the U.S.!

A significant component of specialization in masters- or doctoral-level work is determining what has been done in the fields in which you’d like to work. The emphasis of graduate level work in history is contributing in some way to the field or a particular subfield—whether through an entirely new argument about a historical event or phenomenon, a refutation (even a slight one!) of another historian’s arguments, or even supporting the work of other historians by using primary sources that haven’t really been used before, etc. One way to determine where gaps may lie in these fields is by keeping up with newly published journal articles in your area of interest. There are a number of extremely specialized journals out there, so I’d recommend picking some of the topics you find most interesting and finding specialized journals that cover these topics. You can also go about this by determining well-known historians in these subfields and seeing where they’ve published. I believe in another comment you mentioned that you’re interested in the history of morality—something like The Journal of the History of Ideas might be a good starting point for that particular interest, for example.

In keeping with what the other commenter wrote, it’s also a very good idea to keep an open mind when it comes to your studies and the areas in which you’d like to specialize. I’ve found that my interests have shifted a lot over the years as a result of different historical books I’ve read and various classes that I’ve taken. Your work in a history program will set you up to be able to analyze a wide range of sources across a wide range of historical topics. Don’t be afraid to change your interests over time! Given how interdisciplinary history is as a field, your shifting perspective over time can be a really great asset in terms of contributing to new ways in the field.