It won't necessarily harm it, if you let it 'seep in' for maybe 30 minutes or so. BUT, make sure you do wipe it down and/or clean the blade before too long. Fruit acids will generate rust (pitting, more importantly), if left in contact for too long. I've let mine be, after slicing fruits & such while eating a meal. But after I've finished eating, I'll clean the blade, usually with a Windex-moistened clean paper towel.
David
after cleaning the knife, just a light coat of mineral oil? thanks