I encountered Miura Knives somewhere while I was home. Maybe a program, or reading about quality knives. Don't remember exactly. I learned about Rockwell scales, and AUS10, and other details. But I made a mental note. Managed to visit them briefly while in Japan, but not enough time to really shop. So we didn't get one that visit. I was planning to go on a future trip, but 1poorkid beat me to it.

The blades are just steel. Not stainless. So, as soon as you use it, you should wipe it. Don't let food or water sit on it. Otherwise you will get the patina you described because it is oxidizing (i.e. rusting). Especially pay attention to acidic foods like onions. Cut them, quick rinse, and towel dry immediately.

The oxidizing will also affect the cutting edge, causing accelerated dulling.