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The new fad, trend, or style of the moment.

Background: Since black is always in style in the fashion industry, saying something is "the new black" means that it is the hottest new thing. This phrase can be used to call out the "coolness" of anything, it's not just limited to the fashion world
"Striptease aerobics is the new black."

"I just love your yellow purse. Yellow is the new black this season."
by Sarah February 15, 2005
Anything that is now popular & 'in'. Its usage goes beyond the fashion world where the color Black is always in style.
"Obama is the new black" (Pun intended, no disrespect to African Americans)
by myERa January 02, 2010