They are using what's called (DTG) "Direct To Garment Printing". Basically they are "printing directly onto the t-shirt".
The Printing on "Black Shirts" is a work in progress meaning it's not perfect. Which is why the other poster mention the white from one shirt to another varied as far as brightness.
The in-consistency is due to the fact the shirt much be pretreated before white ink can be printed on the shirt. The method for pre-treating shirts needs to be perfected more. Hopefully, within a year or 2 the ability to print "white ink" on a dark shirt "without human intervention" should be possible and thus improve quality of "Black Shirts" using the DTG method.
Cafepress is definately improving their black tshirt printing, although it is still in Beta testing phase. They should have it perfected soon. Then they should be adding other different colors shortly thereafter...
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