yeah, all the blank t-shirt brands mentioned here are all (for the most part) made for the imprint biz. So they use high quality fabric dye, black, brown etc... that will not burn off the shirt when using a heat press, for example.
One problem you will run into when using cheap t-shirts or t-shirts off the shelf from non-imprint type brands, sometimes the dye burns off the shirt when heat is applied. Its sorta looks like stains appear out of no-where on the shirt. The stain look is acctually the cotton color " Greige " that comes thru on the spots where the dye burned off.
This problem is also something one should look into when starting their own blank t-shirt line etc...
Test the Fabric before you manufacture the shirts.