Your remaining squash is likely also doomed, by what I doubt anyone can tell. Is this happening now in North America somewhere? Maybe it's season has simply passed.

Do never store such a fruit!
Take care, Aspergillosis is a serious danger to your tubes and to your lung.

Thanks for the advice.
I had another fruit that also developed a couple dark spots which on the previous pumpkin, grew into ever-larger black indentations that eventually rotted. On a whim with this remaining fruit, I made a concoction of dishwasher soap, vegetable oil, and water then washed the fruit 3 days consecutive thinking that there's nothing to lose. The black spot disappeared and the fruit has grown larger! Now the challenge is to get it to ripen. I can only assume the black spot was some fungal infection?

Here I show you a fruit of Cucurbita ficifolia, which has a very hard shelled skin. This armor makes cleaning from Aspergillus niger possible, when other squash and pumpkins already are infected in the flesh. Your variety of C. maxima kabocha group was bred with C. ficifolia long ago in Asia for that reason of having better tolerance against fungal diseases and many other pests including fruit flies.