Acronym for Blue Sky, Black Death... Skydiving slang. It means you should enjoy the blue sky around you but don't forget the black earth below.
It is generally used by skydivers in place of "RIP" for those fellow skydivers that have passed on.
"Hey man did you hear the awful news? Josh had a fatal accident while BASEjumping the other day. His chute got caught and failed to open"
"Aww man, that's so sad. He was such a great person. BSBD Josh. You will be missed"