It could very well be an allergic reaction. I found this info online;
"Close inspection of the underbelly might reveal pigment changes, either widespread or localized, often rust-colored, sometimes charcoal gray or black. Sometimes the belly skin shows tiny pinpoints of pigment that look like blackheads, but are not. This is different from "flea dirt," which washes off".
Here is the website;
Dr. Adelia Ritchie Discusses Dealing with Dog Skin Problems
Hope this helps.