I solve this black paper dilemma at least once per week in the college darkroom. Most times a student puts a sheet of paper in the easel either while the enlarger light is being projected or when they are focusing the image on the paper. Then they turn the enlarger off and give it the brief test exposures. The prints have white borders where the easel masked the paper but the rest is black. I don't know if that is what you are doing, but I see it enough to mention just in case.
Joe
Joe