Your Adenium looks to be in good health. Do you remember this being there when you re potted the plant, assuming not from your comment? With the sap running from wound, it could be that it got damaged. The color of the tissue is fairly dark and definitely worth keeping an eye on it to make sure that this area does not become larger. Coloration could just be the skin healing. With gloves on (to safe guard sap on skin) lightly squeeze the trunk/stem and make sure over the next week or so that it is not becoming soft. This would mean pruning it, if this was to happen.
Rick
