Do you mean heat transfers from transfer paper, or transfers in general, including, like, vinyl and plastisols? Because people also use those as transfers on black hoodies/sweatshirts.
A plastisol is your design "screen printed" onto a transfer release paper, instead of directly onto the shirt itself.
It is made from plastisol ink (just like ink plastisol screen printing). You can order these from companies that will make your design for you, and they will mail them to you on the release paper. When you have an order for a large sweatshirt, you take your plastisol transfer, heat up your press, and apply the plastisol to the sweatshirt.
It looks really nice. Someone posted a pic of a sweatshirt done with plastisol transfers not too long ago. I wonder if you search "plastisol sweatshirt" or "plastisol sweatshirt pic" if it will come up.
Does plastisol sound like something you would be intersted in?
Heat Transfers for Black Sweatshirts 
