Greetings . . .
As I understand it all Chevy engines were painted gray until the full pressure oiled design appeared in '53. In that year the solid lifter 3-on-the-tree 235 cars had gray engines and the 235 power glide cars had blue engines. This was the first year for the so called Blue Flame six:
So a '51 235 would have been gray from the factory.
regards,
stock49