If you block it... the blowby would likely blow the dipstick out, and squirt hot oil on the engine bay. In a CA18 oil pumps straight onto exhaust manifold. Makes a great engine bay fire on the dyno. But this only if you block all breathers.

The A-series also have a breather on the back of the engine.. so it would be unlikely to cause much problem under normal driving. If you connected the two breathers together .. you would have dipstick action