Let's start with Black Flag.
Black Flag is considered by many to be the first band on the hardcore punk scene (evidently there's punk and hardcore punk). Greg Ginn founded the band in southern California in 1976 and remains the only member to stay in the band throughout it's 20 years. In all, 15 different people would participate in one way or another. The most notable and famous to date was singer Henry Rollins, whose shorts-only performances became his signature outfit (no shoes, though... how on earth do you make a signature outfit without shoes!?)
The band was named Black Flag which was used to represent anarchy. Raymond Pettibon, brother of Greg Gin and bass player for a time, stated "If a white flag means surrender, a black flag represents anarchy." I believe a bug spray of the same name had an influence as well.
Black Flag had a do-it-yourself type arrangement. The recording, PR (that's public relations, not Personal Record), advertising, tour managing, etc was all taken care of by members of the band.
Their sound is loud (too loud for most of you - let that be my warning), with screeching vocals and atonally manic guitar solos. The band had a high vocalist turn over - quite frankly the human voice cannot screech and yell for very long and each singer went horse and dropped like a fly. Most of their songs are about insanity, paranoia, alcohol/drugs, violence... great stuff for the kiddies. Here's a sampler: Nervous Breakdown, TV Party.
And then there was Black Sabbath (well, they actually came before Black Flag but you gotta end with the best).
The creation of Black Sabbath began in 1968. The band, located in Birmingham England, was first named the Polka Tulk Blues Company and later called themselves Earth. Primarily, they were a blues band but decided to add a little heavy metal to both their sound and their name.
If you think Black Sabbath is about black magic and the observance of darker religions, you are correct. Geezer Butler, the bassist, was reading a black magic novel and had a vision of a black hooded figure in his sleep one night. Butler told vocalist Ozzy Osbourne and a new name was found.
The trademark Black Sabbath sound was discovered when members of the band were sitting around (knitting I'm sure) and realized collectively that people paid money to see movies that scare them - why not make music that scares too!
Black Sabbath has a very long and complicated past and many musicians entered the band and left. In 1979, Osbourne was asked to leave. They say it was because of a drug problem and lack of commitment. My theory however, is communication, not commitment. No one could understand a dang thing he said! After Ozzy was given the boot, he went on to have a fantastic solo career eating bats on stage and later starred in his own reality TV show called the "Osbournes" - where basic mundane tasks like emptying the garbage can are now entertaining.
When Ozzy left, Black Sabbath replaced the singer with a vocalist by the name of Ronnie James Dio. The band experienced a fair amount of success and Dio invented the "mano cornuta" or "devil with horns" hand gesture that is so popular among hard rock concerts. (I wonder how the cell phone, which has since replaced the cigarette lighter will change that?!) Check out Paranoid and Iron Man and TV Crimes (with Dio).
Happy Black Friday