Ed - "Dominant Black" It's a full extension of dark pigment; it brings the black (or blue, chocolate, or lilac), all the way up the hair shaft (which, by the way, is hollow). (Ed) will make a chestnut coat look black, an opal coat look blue, and so forth. The "blacks" brought on by this color are very deep in color and are less likely to have scattered whites than a true black. Things may change someday, but for now, (Ed) is rarely found, so it's safe to assume your black (or blue, chocolate, lilac), is a true black (or blue, chocolate, lilac), unless it meaninglessly throws (produces), agoutis when bred to a true self.
Es - "Steel" This is a lesser version of (Ed). (Es) carries the dark pigment mostly all the way up the hair shaft, but leaves a little light color at the tips of the guard hairs. This produces the steel variety when coupled with the agouti gene. Since self and tan patterned already have single-colored hair shafts, there's no light color at the end of the hair tip for the steel gene to leave. Thus, the steel gene doesn't show on (aa) or (at) patterned animals.
Have a nice day!