V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

RBL
       Realtime Blackhole / Block List (Internet, SPAM)


The Jargon File (version 4.4.7, 29 Dec 2003):

RBL
 /R?B?L/

    Abbreviation: ?Realtime Blackhole List?. A service that allows people to
    blacklist sites for emitting spam, and makes the blacklist available in
    real time to electronic-mail transport programs that know how to use RBL so
    they can filter out mail from those sites. Drastic (and controversial) but
    effective. There is an RBL home page.