Yes to specializing on a niche type of music

It sounds like part of your question is “what blog should I start?”. That expands beyond just SEO and is more of a question of “what is a good business idea”

For that, as you said, definitely do some research. But really just see if there’s an unfulfilled niche. Are people asking the same questions on forums? Are your friends/colleagues asking similar questions? If you’re fulfilling a need for a content site that doesn’t exist, then that will likely be the first thing and SEO will follow