I read this on one of the other boards the so-called joke links to...

The "blonde joke" is a subtle method of linking dozens and dozens of internet fora, providing multiple paths along which our merry spam-bot can happily wend his way, splattering each forum with a plethora of crap, before moving on to the next via a convenient link.

Their board was attacked by a "porno spam bot" 24 hours after the blonde joke thread appeared...