Wikipedia pegs them as "heavy" or "speed".
But really, I don't think they were apologizing to the band, I think they were apologizing to fans. The band likely didn't care too much, since they were already appearing on the episode being pretty silly in the first place.
I love how serious the fans take themselves though... This quote from the linked article is perfect:
if you brazenly mix up black metal and death metal – and the essence of the joke hinges on getting that reference right – well, that's just lazy writing and it deserves to be called out and mocked relentlessly
No, the essence of the joke really DOESN'T hinge on getting that reference right. If anything, it's a little bit funnier that a clueless FBI agent gets it wrong, and it's something that only fans of that genre of music will get any way. For everyone else, the joke hinges on the band being a metal band, period, and changing their song lyrics from "Breaking the Law" to "Respecting the Law". It doesn't matter of they're a death metal band, or a black metal band, or a speed metal band. To the VAST MAJORITY of Simpsons watchers, nobody gives a flying crap.
Edit because I forgot to add (which I originally came in here to say):
I totally did not get the reference from that chalkboard joke, even though I had just watched the offending episode less than 2 days before watching the new one. Goes to show how much attention I'm paying when I watch these days, a far cry from the early days when I can quote you entire episodes.