How many curves you are using to loft ? It looks like you are also revolving? The black area is the normals are reversed. it is either a curve direction is reversed. Make sure that all the curves starting points are in a straight line.
If you select the surface which is black you can always go to Surface Menu and at the bottom you will find Reverse Surface Diction..... that might help too.
Tried rebuilding the curves?
If you upload the scene , May be I will be able to find out what is wrong
Use rebuild curves options to clean up the curves so the match each other better.
If all the curve directions are the same. And you don't have "auto reverse" turned on, then have you confirmed that the curves are lofting in the correct order? Did you select the curves sequentially or all at once? I would go into either the node editor or the hypergraph and confirm that the curves going into the loft node is in the correct order.
So there's a bug in 2020.4?