I understand what you mean.
This year our first room in our extension was a mask room with all white maskS hung by black threads and 2 guys in black costumes that would hold some mask and make certain ones jump out at people.
The only problem was the change in day light saving time.
It didnt get dark until 7 so I didnt think the room would work for the first hour( we opened at 6)
I solved the problem bY sealing all outside light possible, and then covering the 20inch black light allowing only about 1 inch of light to get through.
The masks light up great and the room worked even in bright sunlight outside.
Try dimming you black light or only allowing alittle light through, It worked for us.