breeding black mexican king snake to cali king?

[Brandon Osborne]

I suppose, though aren't they still 2 different species?

Nope. They are different sub-species. Lampropeltis getula california and Lampropeltic getula nigrita....thus making them an integrade. A hybrid would be if you crossed say L.g.nigrita X Pantherophis guttattus guttattus, aka mexican black king x cornsnake.