Unfortunate as it is, sublimating straight to dark clothing is still a concern. This reason for the concern is that the substrate (in this scenario, the dark-colored garment) is made of fabric, and the fabric has already been dyed to achieve the dark color.
If you are looking to sublimate, say, a red, dark pink, or orange item, I would highly recommend using a darker color ink for the transfer (such as black) to ensure the transfer is visible. This way, your transfer will show up against the dark substrate (garment).
If you do not mind a bit of a hand to your polyester garment, there are many suppliers that sell a two-step, sublimatable patch product. This product allows you to sublimate your design to a flock material and then transfer to a dark-colored garment.
—Coastal Business Supplies