In some cases, the dark steel agouti may appear completely black, or just have a patch of steel coloring. In the case of a broken, it's feasible that the steel coloring may be so dark that it may not be noticeable at all.
The only way possible to get an agouti is to have an agouti parent. We know that the otter isn't, so the only other explanation (if these 2 are indeed the parents) is that the "black" is actually an agouti steel and not really a black.