they weren't aub-Saharan African in general but there were doubtless some citizens from Caracalla's time onwards who had some sub-saharan African blood and even East Asian blood
The black and white dichtomy is daft, the Romans only cared if you were a citizen or not.
I've read reports that first the Greeks and later the Romans made trips to the Canary Islands to obtain slaves there because the had a Nordic appearance, tall, blonde and White and they were the preferred sex slave.