Black cracks, just like all the rest, if you over-tighten your flush nut.. Just remember it’s the owner that cracks them. They don’t simply crack, while nobody is looking.. One colour being less prone than any other, is a modern urban legend.
I’ve a transparent black item. No signs of cracks, and it’s in constant use.
Incidentally. Flawless will have a copy of your receipt. This should cover you, for this problem. Just tell them the date of original purchase, and your personal details. It’s even smoother if you paid by credit card or PayPal.