Not only can solid black be used, but also images that will repeat endlessly on the receiver's machine until its toner runs out.
[5] The introduction of computer-based facsimile systems (combined with integrated document imaging solutions) at major corporations now means that black faxes are unlikely to cause problems for them.
On the other hand, the ability of computer modems to send faxes offers new avenues for abuse.
A program could be used to generate hundreds of pages of highly compressed, pure black – or huge volumes of relevant-looking, original, non-repeating high-black-density junk, just as effective but far more difficult to counteract – and send them very quickly to the target fax machine.
Black faxes are similar (in both intention and implementation) to lace cards.