the blackscreen bug, as frequently reported in the quest series with brittany, for example, is relatively easy to fix without restarting the game. simply load the last savegame and go through the customization of the npc's in the mc's room on the pc. if the character has an invalid change, you will immediately get a black screen.
perform a reset beforehand on a character that was working and mark the position and order of the buttons on your monitor at home (no permanent marker). since the game continues to run in most cases when you get a black screen, you can now load the broken character and simply click on the marked positions there. voilat. reset performed. you can now enter the park again. this might not work with every error, but it worked for me. best of luck.
