“Denim” comes from the French phrase “serge de Nîmes”, or fabric from Nimes, after the city where the weave was popularized and probably developed. It is similar to, but not the same as, cotton duck, which is a traditional fabric for sails. If you look at denim, you can see a diagonal pattern to the weave which you won’t see with duck.
Cotton duck is the stuff that cloth sneakers are usually made of.
If either denim or duck was made of undyed, unbleached fibers, the result would be somewhere on the beige to tan scale, depending on exactly what kind of cotton it was made from. Denim is traditionally made from bleached fibers where the warp threads (the ones that go up and down) are dyed with indigo to make them dark blue, while the weft threads (the ones that run side to side and are woven in) are left white. Because of the style of the weave, the resulting fabric looks blue. But if you’ve ever torn or cut your jeans, you’ll have seen the white fibers working loose.
Denim refers to the style of weaving, however, and not the color. It’s just as much denim if it’s dyed gray or black or hot pink.