There is nothing wrong with sublimating on dark fabric, but the results may vary depending on the fabric. For example, you will get a different result by sublimating from white to black because more of the dye will be absorbed into the dark fabric. Also, the sublimation process may take longer since the dark fabric will require more heat than a lighter fabric to soak up the dye.
It may take a few minutes to sublimate over a dark fabric, so ensure you watch the progress of the transfer. Once you begin to see the sublimation transfer process working, then you can increase the heat accordingly.