Black ice often forms in cold-spots on otherwise warm roads; places such as shadows, dips where cold air collects.
It forms on untreated cyclepaths alongside roads that have been safely gritted.
At near-zero C (ie freezing point) black ice may have a layer of meltwater for extra concealement and slipperyness.
There are other kinds of ice on the road, but black ice is the most hazardous. If you ride with any dependance on tyre traction (ie lean, steer, road camber, slick tarmac, brake or balance recovery) you are going down fast.
Studded winter tyres are a solution but black ice can sneak in on otherwise clear and cold mornings when you might not be using studs. If your winter is long, cold and icy, you should be using studs the whole time. In more temperate climates, cold days can be exceptional and no-one wants to use studs if they don't have to.

Black ice may form on rough surfaces (unworn chip and seal) but this presents no real danger. You need a smooth surface to be really dangerous.
Note the micro-climates on your route esp cold spots where black ice can form.