I was aware of their existence for a while before the Bundy Ranch incident. However the were always low key, never head of armed (well maybe they had a shot gun for defense against wild animals in the trucks) BLM agents before Bundy.
They have been armed since the 1970s...right after the FLPMA became law.
“The BLM launched the CDCA ranger program after the passage of FLPMA. BLM rangers were then trained at the Federal Law Enforcement Training Center in Glynco, Georgia.”
http://www.blm.gov/wo/st/en/info/history/sidebars/law_enforcement/the_sagebrush_ceiling.print.html