I have shot with a hammer gun for longer than i care to think - and it has never been a problem i have been aware of - but i shoot with 2 eyes open.
This first photo shows the picture from a right eye - right shoulder.
And yes you are right - the hammer - either cocked or down will obscure part of your vision. As you can see i have purposly focused at the distance like you should when shooting a shotgun.
This second photo shows you what your left eye will see and when you brain combines the two images the hammers will become transparent and the problem goes away - another good reasion to shoot both eyes open.
But only if your shooting shoulder is next to your dominant eye!