The patina is controlled (somewhat) layer of rust.
It helps protect the blade from other corrosion and some like the looks as it's more even and a little less care in the long run.
However, a patina can lead to discoloring of food as you cut it, leaving dark smears on something like cheese.
Methods, some will sitck the knife blade first in a glass of vinegar so they can watch the blade darken. On a long kitchen blade, that's not so easy. So you would wrap it in paper towels saturated with vinegar. You want a tight even wrap with no gaps or the patina will be gappy. Check it every 15 minutes or so to see how it's going.
I like the potato method too. Stick the blade in the center of a large potato. For a large kitchen knife, you'll need a number of big potatoes, with the ends trimmed so you get a good even fit and coverage of the blade. Harder to see what's happening though. This is slower than the vinegar method.
Some steel take quite a bit more time. On a 1080 blade of mine, it happens quickly. On an A2 blade it took hours. On a friends D2 blade, it happened cutting a strawberry. Of course, surface finish enters into the time question too The A2 blade has a high polish.