they are blocked because they're not used.  a SBC head (or
derivatives) will work on either side of the engine, there aren't
right or left side only heads.  if you look at the intake manifold,
there isn't a coolant passage in the rear, like there is in the front.

hth,
Bret

Water flows through the front and not the back.  The heads fit either
side so the back port is blocked on both heads when installed.  Check
the intake and you will see that the water runs across at the front to
get to the thermostat opening.