The term black fax refers to a prank fax transmission, consisting of one or more pages entirely filled with a uniform black tone. The sender's intention is typically to consume as much of the recipient's fax ink, toner or thermal paper or disk space as possible, thus costing the recipient money and/or denying the recipient the use of their machine (this is similar to computer-based denial of service attacks). This is made easier because the fax transmission protocols compress the solid black image very well, so a very short fax call can produce many pages.
The basic principle of a black fax can be extended to form a black fax attack. In this case, one or more sheets are fed halfway through the sender's fax machine and taped end to end, forming an endless loop that cycles through the machine. Not only can solid black be used, but also images which will repeat endlessly on the receiver's machine until his or her toner runs out.