Looks like a rot or fungus to me Joe.
I'm seen mulberries that were eaten up by something making the wood not so great. I don't know if there is some disease they get, or what. I'm noticed it though. I have a stave from a tree I cut, the mulberry tree has something crazy going on with it. The sapwood didn't seem to even be there. When I finally got to look at it closely, it was a very thin layer of sapwood, and it was discolored dark too. I thought I had found the rare species of mulberry that comes with the sapwood already removed!
The wood looks kinda airy and not so great, but it is a pretty heavy stave still for mulberry, so I figure sometime I will try a bow from it.