I'm still new to snakes as you know but I would say from looking at King snake pics,and my Marv, they don't look like they have the same shape head or neck.
Looking at rat snake pics, I would say yes, part rat snake......but I could be wrong lol Just doesn't look king like to me.
looks like a black rat crossed with a gray banded king to me! people cross ratsnakes with kinsnakes all the time, corn x cali king is a popular one! the 2 species are more closely related than most people think. not sure where i stand on the subject, but its a nice looking snake all the same mate!