Properly done, the patina is an oxidation of the outer surface of the blade, and minimizes the ability of the surface iron from further reacting to any strong acidic or basic foods or food juices. It builds up over time and is perfectly acceptable and to an extent, desirable.
No, it will not wash off. It can be scrubbed off, and if you want to minimize it, then baking soda, green nylon scrubbing pads and elbow grease will do that job (I would suggest you only use BKF when you really need to bring a knife back to bare metal).
Yes, you need to wash off the knife after you use it, IMMEDIATERLY. Not "tomorrow, some time", not "after the meal", not even "ASAP". IMMEDIATELY means IMMEDIATELY, .And after washing, the knife needs to be wiped dry, then left where it can air dry further to allow the final moisture film to evaporate. Only after that can you safely put the knife away.
Please note that I used the word "minimizes" above about the patina as a protective layer. Nothing is an absolute protection (even to "stainless" steel). Given time, a dirty knife can end up with the food residues breaking down into acids and eating through the patina to begin a rust spot on the blade. That rust spot will invariably have the end result of pitting the surface of the blade - definitely a bad thing.
Which bring up several more basics
Good knives should never be put into a dishwasher - hand washing (preferably with hot, soapy water and moderate scrubbing to remove food residues, followed by running water rinsing) only, with immediate towel drying, followed by air drying.
Never, never, NEVER leave your knife in standing water.
Just simple procedures, repeated until they are reflexive.