CA issued black plates with yellow lettering in '63, replacing the previous yellow plates with black lettering.
Late in '69 they changed colors again to blue with yellow lettering, like what I see on the truck's ft bumper.
Only way that truck could have black plates would be a personalized black plate moved to the newer truck, which would be extremely unusual in that time frame.
Personalized black plates are extremely rare, and the CA DMV has always been difficult to deal with as far as plate swaps.
I remember my Dad having to turn in the yellow plates for the black ones.