In order for FH+ to reall work you are going to have to take a character from the regular Feral Heart game to Bonfire. Its the only way to really access this feature in FH+.
Reason for the all black screen is because for FH+, places like The Temple of Dreams, or the Plains dosnt exist in that feature.
What i would do is go on your regular Feral Heart game, and travle to Bonfire. Once there, exit Feral Heart and click open FH+. From there you can access the character you put there and travle to other maps from there. Reason for the black screen is if you ever really have one, is cause the file for it dosnt exist in that game. FH+ and Feral Heart have there own seperate folders.