I've been working in SQA field for a year now, and basically I used only black box methods. What advice can you give me to enhance my skills?
Recently I have started learning Jmeter Apache for load testing.
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Based on my opinion, always depends on your skills. I mean, if you are testing Windows desktop application, it's highly recommended some .net knowledge, but if you are running Android tests, Java is the base of the knowledge.
Black-box testing is quite a general way to describe a testing method, it can include a lot of technics, like a boundary test for example.
If you don't want to focus on the development part - on the "pure" code, focus on the other aspects of the application, not strongly related to its functionality - on the non-functional parts: usability, security, scalability, stress testing. The static testing - rereading and testing the documentation is quite useful, and can prevent many possible bugs and failures from the application.
I would highly recommend to have a look on the V-model, I think, it is useful and a tester can learn many aspects even if the project development doesn't follow its path.
There's a lot of ways you can go here - if you want to stay with manual testing you can look to improve your ability to find and report problems, or you can add load testing or functional regression testing to your skill set.
Depending on where you're based, the most effective method for you could vary - you'll want to balance between something you enjoy doing and something that is in demand in your part of the world.
I'd suggest you read widely. Joe Strazzere's excellent blog, All Things Quality, is a great starting point. Another good place to look is the Ministry of Testing for an excellent list of tools of all sorts as well as a consolidated blog feed.
Since you have used basically black box methods, you may want to widen your QA knowledge by introducing yourself in topics like code inspection, code analysis, technical debt, code metrics... This will allow you to predict which parts of the software are most likely to fail and which parts will require more effort to be maintained or evolved.