Doxy eggs are black in colour illness-inducing to humans when ingested. Fred and George may have sought to exploit this property in the development of their Skiving Snackboxes (OP6). It also caused Cormac McLaggen to miss Quidditch tryouts during his sixth year after he ate a pound of doxy eggs for a bet (HBP11).
The doxies that had infested 12 Grimmauld Place had laid eggs (OP6).
During Molly Weasley's cleaning day at Number 12 Grimmauld Place, Fred and George were trying to sneak away doxies and their eggs because they were "experimenting with doxy venom" an an ingredient for the Skiving Snackboxes of candy that could make people too sick for class.