Dark enough for fireworks about an hour after sunset. For the Boo Bash dates, the latest sunset -- the first date -- Is 8:09 pm. Really, they could probably do a 9:00 pm or 9:15 pm HEA. But that would be more consistent with a 10pm Boo Bash start, not 9:30
So yes, 9:30 pm for the first few Boo Bash nights could suggest 9:00-9:15pm fireworks start times.
For example, Saturday August 11, 2018 had HEA at 9:15 pm but After Hours began at 10pm
August 8, 2019 -- HEA at 9:00 pm, Villains night starts at 10pm
August 1, 2019 -- a pretty late sunset (8:16 pm), they HEA at 9:15 pm and After-hours started at 10pm
So looking at the history -- 9:30 Boo Bash really isn't consistent with fireworks. If it was a 10pm start time, then I'd buy it.