Red Belly would be a safe bet.
The skin is very dark, so the snake would be or very close to black, but a tell tale sign is the two tones in the ventrals.
Look at the sides and see the black? Most RBBS have black on their ventrals (some more than others, a recent thread showed a RBBS almost completely black. All my RBBS ar 50/50) The clearer parts on the skin will be where the belly is red.