I'm pretty sure CafePress has created options for darker shirts now. They may be in beta or something, though.
I don't know if it's improved, but they had some quality issues with making the logos show up bright enough with the black shirts. I held out on getting a shirt for a podcast I liked until they made black available, and true to their warnings, the logo is kind of muddy and indistinct.
Spock does not approve.