This is a complicated, often confusing subject and I'm hoping to shed a little light on it. Here's a timeline that might help:

Keith Morris - 1977 to 1979
Ron Reyes A.K.A. Chavo Pederast - 1979 to 1980
Dez Cadena - 1980 to 1981
Henry Rollins - 1981 to 1983
(NOTE: Black Flag technically existed until 1985, but they did not release anything during those last two years other than the great live album Live '84, and Who's Got the 10 1/2, another (although much shorter) live album.)

Black Flag was formed by Greg Ginn and the first vocalist, Keith Morris. Keith left because he was basically sick of taking orders. He went on to form the Circle Jerks. Material featuring Keith can be found on Nervous Breakdown and Everything Went Black, a compilation featuring all the pre-Rollins vocalists on one CD. He is also on The First Four Years, another compilation of early material.

Keith was replaced by Ron "Chavo Pederast" Reyes. Chavo quit two songs into a show at the Fleetwood and walked offstage, leaving the band to do their rendition of "Louie Louie" for an hour, with anyone who wanted doing vocals. His stuff is on Jealous Again, The First Four Years, and Everything Went Black.

Next up was Dez Cadena. He was a stable vocalist, and didn't have any conflicts with the band, but he had to quit due to the serious strain touring put on his voice. He continued to play guitar with Black Flag until 1983. His vocal work can be found on Six Pack, Everything Went Black, and The First Four Years.

And finally there was Henry Rollins. After jumping onstage with the band in New York, Dez called him and offered to jam. Soon Rollins was Black Flag's singer. He released a large amount of material. Here is a list of all notable recordings:

So there you have it. The history of Black Flag's vocalists. Black Flag was a brilliant band, and I highly recommend checking out any of their material - it's all good stuff. Many feel that the later work with Rollins was weaker, I think it's excellent. It is different, but that's expected - a band has to evolve. Pick up The First Four Years, Everything Went Black, or Damaged, and hear one of the best punk bands ever.

Thanks to http://www.ipass.net/~jthrush/flagline.htm for providing some of this information.