With death metal, it is the brutality combined with the levek of control. It creates a really impressive display because even though the musis very extreme and aggressive, there is a strong level of tighness and technicality. Also, the lyrics in death metal are often very shocking but also well written and structered, so the appeal there is comparable to that of rap.
Black metal is a little more complicated. Most people who are really into it tend to be very pretentious and elitistic about it. They usually need a cause but can't find one so the talk up the genre like it is some serious spiritual thing. It also appeals to rebellious individuals because it tends to go along with extreme mentalities (usually mostly image tbh).
I'm not trying to shit on black metal because I like quite a bit of it...there are legitimate reasons for appeal-it has a very "rustic" quality to it because of the poor production and some of it is emotionally powerful.
As for good vocals, anything that stands out is usually good provided that it isn't bad. For good death metal vocals, look up Massacre's first album. For black metal, listen to any of Bathory's black metal albums.