That’s to a fairly substantial degree deliberate, especially with regard to death metal and black metal.
For the most part, what’s being said doesn’t matter nearly as much as how it sounds and how it feels (and looks, depending on the band). That’s true of a lot of song lyrics and especially true of death metal, most of which is written with tongue thoroughly embedded in cheek. Your average death/black metal band member does not have a massive well of horrific negative emotion to draw from, after all. And yeah that philosophy extends to band logos, the illegibility of which is something of a meme at this point.
The unreadable logos actually bother me, as someone who likes the genre. You can do the gritty dark thing without making your logo hard to grok and therefore less memorable. Readability was one of the only design constraints I put on my friend when he was designing our logo. I wanted people to be able to find us on Google after seeing a poster or t-shirt with the logo on it (and obviously, to want to go looking).
