2 reds cannot produce a black. Did he come from a big kennel where there may have been an accident? Don't want to upset you, it's just a genetic fact. You'll love him no matter what.
Those edges look very clean and healed, if it's coming from inside I don't think it has to do with the ear trim. Big red flag would be a bad smell (that means infection). It IS kinda nasty looking though. You might take a flashlight and look down by the post to see if it's coming from underneath somewhere. If it is coming from under the posted area/tape you should take it off & clean it up and find the source.