It’s never occurred to me to wash a can opener.
To my knowledge, our Daisy wall mount can opener was never washed when I was a kid. Black crud would periodically peel off the blade when I opened a can.
As a matter of fact, that’s what prompted this question.
I opened a tin of smoked mussels – rectangular – and black crap came off the blade of my manual can opener. After I finished eating the mussels on saltines (we found the mussels in an old picnic basket and they were chalky, making me afraid they were bad) I asked this Q.
In my defense, I recall washing a can opener in the past and seeing rust on the cutting wheel. That scared me worse than the crud. I have this childish (as in child like or formed in my childhood) notion that rust will give me tetanus.