Hellrider

FA said it much better than I could ^
I would just add that also key to Black Metal is the aesthetic of the music and the feelings it connotes, which include but are not limited to: sadness, transcendental thoughts, spiritual beliefs/blasphemy (namely in the name of Satanism or various forms of Paganism), pain, depression, the melancholic essence of winter, and admiration for the strength of nature. I know that's not the most coherent explanation, but those subjects are very important to what I perceive to be the "Black Metal aesthetic".

The best way to do that would be to listen to a bunch of black metal and figure it out for yourself. Stereotypical black metal based on the early 90s Norwegian sound has blastbeats, tremolo-picked guitar parts, high-pitched screaming vocals, lots of reverb, and often shitty recording quality, but none of those ingredients is essential. We have a recommendation thread in the black metal subforum with a lot of music for you to peruse.
You'll get some purists insisting that black metal is only "real" if it's Satanic, which I disagree with, but that's a different conversation.