Wok preparation and caring

I don't think those are "cracked ridges"; I think you have a hammered wok. The appearance is an artifact of how they are manufacted, by hammering a steel blank into a form or mold. It probably is a carbon steel wok, as that is the most common material used for the hammering method as far as I know.

According to this article at The Kitchn, you should season it as follows:

1. Scrub your wok in hot, soapy water and then dry it over low heat on the stove.
2. Turn the burner on your stove to HIGH and add 2 to 3 tablespoons of oil to the wok. I prefer peanut oil, but any oil can be used - sesame, chili, or canola. Coat the wok surface evenly with oil by tilting and turning it.
3. Heat the oil and keep the oil spread across the surface until it starts to "glaze" the wok - in other words, it turns into a thin layer of film. Use a wok spatula to move the oil around and tilt/turn the wok to move the oil as well. Be careful not to spill any oil that would start a grease fire!
4. Let the wok cool and the layer of oil will harden on the surface.

As long as the surface isn't rough (i.e. with burned-on bits of food) then yes, the patina should be a dark brown or black.

That's more-or-less what a patina is; it's essentially tarnish, the result of cumulative oxidation, which normally happens to some metals anyway but is accelerated by the rapid oxidation of oils via high heat.

If you end up with a slick, evenly-distributed black layer, then you've seasoned it perfectly. If it's streaky or splotchy then you need to start over by scouring it down.

(P.S. Carbon steel is different from, for example, cast iron, because the latter is porous and you'll never see that perfect black, just a noticeable darkening.)