If you're just starting a blog/technical publication, there's nothing wrong with casting a wide focus and writing about many barely related topics. As you get some articles written/published, you'll learn more about what you're good at, what you enjoy and what brings the most useful traffic.

At this point, you'll be better served by building a publishing habit than picking the mythical perfect niche. But, if you're concerned, in my experience, all the good niches either have huge amounts of competition or they're so incredibly bleeding edge that they're risky to learn about.