There are a couple of things that could cause this and we will need some more information to find out what is happening:
It could be that your Layer is less than 100% opaque. Please check the Opacity value using the menu arrow for the black-filled layer in the Layer Panel, that should indicate the value. If it is not 100%, set it to 100%.
One other possibility is that your image was not saved as a Painting file but exported then re-imported. This will change the colour of the paint as exporting applies a light to the canvas so that other applications can see the texture of the paint applied.
The black area isn't quite black, it looks as if it is a really dark version of the canvas colour, so I am guessing that opacity is the problem. If it isn't, please send a copy of the painting to email@example.com (if it is less than 5 megabytes in size) and we can take a look at it directly.