Why Design for Black Tees in Mind?
Black tees are the most popular and most requested out of all the other color tees industry wide. The least popular being Lime Green. On the bar graph chart provided by NeatoShop, Out of 3000 shirts sold 48% were Black. (If you have not yet opened up a shop with NeatoShop, I highly recommend that you do so.)
This is because Black is a classic and the color pairs well with everything. It is a neutral color. It also offers the most contrast, making the designs pop. If you design around a Black canvas, then your print will look better on any darker colored fabric. That is assuming that you are not designing for Black only. (By excluding for example, the Black line work and shading from your design. Thus having the line work and shading whatever color the background is by leaving it blank. If so then any other fabric color will not look right in its place.)
Some reports in the industry state that solid white sells better. This is not data related to the "graphic tees" and "design" aspects. White shirts mostly sell as undergarments and account for the cheaply made "Value Tees" that you find at some of the Print On Demand sites. These sites usually use the inferior Heat Transfer method. They are essentially throw away tees as you wear them once and then the design is ruined after the first wash. The price is the main factor there. The statistics for "Preferred Color" is still Black.
Those are the reasons why I would recommend designing for Black Tees. Just because you design for Black Tees doesn't mean that you can't offer the same design on other colors if you do it right.