Years ago in the days of typewriters it meant that a copy was sent to someone else but the original recipient was not made aware of this,  as opposed to cc which meant the recipient was made aware ("blind carbon copy" as opposed to "carbon copy").  I don't know what the letters mean now but it amounts to just the same, the recipient is not told you have sent a copy to someone.  You can use this so as not to divulge emails addresses to other peoople. i.e even the first recipient can be sent it blind so nobody kinows who has received it apart from themselves.