I really don't see the faux pas.

The most glaring "accidentally racist" part of your whole story is the "section with special cards for black people". WUT?

Why is it okay, or even necessary, to segregate greeting cards for people by race? Why are things like this separate? I understand how certain things, like skincare products, might be better off when they're developed specifically for certain peoples, and then marketed that way. But cards? It's been said a million times, but I genuinely wonder what the feedback would be for a line of greeting cards for latinos or white people.

EDIT: Photoshop competition idea. Make greeting cards for different races. First one to get banned gets all of the rep, ever.