From the Retconjuration entry in the Erfwiki:
Rob Retconjured all black/brown to dwagons to purple for the book printing. Story below:
When it came time for Rob and company to design the Real World dwagon plushies, the total number of colors initially came to seven. Pink, red, blue, purple, green, yellow, and black. Rob didn't really want to make seven different colors of plushie, because it's an odd number and it just felt like way too many for a set. This led Rob to see if he could get away with dropping one color of dwagon. After looking, Rob found that black (dark brown?) dwagons had only appeared once or twice, and never in combat. Furthermore, he thought black material would look bad with the embroidered eyes. A little while later, in the webcomic The Order of the Stick, a dramatic story moment occurred: many, many black dragons (not called dwagons in OOTS) from all over the planet were instantly wiped out by a spell. Rob considers OOTS to be sort of Erfworld's godfather comic; this led him to the conclusion that the same spell must have killed off Erfworld's black dwagons. This prompted Rob to Retconjure all black/brown dwagons to purple for the book printing. (Note that nothing was said about Retconning them in the webcomics, and some readers have reported still finding brown/black dwagons in the book.)
Also of note is that immediately above (following the word 'printing'), there is a link where Rob and Xin both recount and illustrate the above story. Currently, it is only correctly visible by those who pledged to the 2012 "Erfworld Year of the Dwagon" Kickstarter.
As a personal note, I find the idea that V's Familiacide spell was powerful enough to reach into Erfworld kind of delicious.